i 


m^8  UBRARY 


THE 

DESIGNING  AND  CONSTRUCTION 

07 

Storage  Reservoirs. 

BY 

ARTHUR  JACOB,  B.  A., 

LATE   EXECUTIVE  ENGINEER  FOR  IRRIGATION  H.  M. 
BOMBAY  SERVICE. 

REVISED   AND    EXTENDED  BY 

E.  SHERMAN  GOULD,  M.  Am.  Soo.  C.  E., 

CONSULTING  ENGINEER  TO  THE  SCRANTON  GAS   AND 
WATER  CO. 


NEW  YORK : 

D.  VAN  NOSTRAND,  PUBLISHER, 

23  Murray  and  27  Warren  Street. 

1888. 


John-'W-Oooam^^. 


'ECEHBLR    27?  18^4 


JPKIA'iF/LLP,   MASS.  - 


EDITOR'S  PREFACE. 

In  preparing  a  revised  edition  of 
*'  Jacob's  Storage  Reservoirs,"  it  was  con- 
sidered best,  in  view  of  the  standard 
character  of  the  work,  to  present  it  in 
its  original  form,  with  all  additional 
matter  separate  from  the  text. 

All  of  the  editorial  work,  therefore,  in 
the  present  volume,  appears  in  the  shape 
of  foot-notes,  and  the  additional  pages 
appended  to  the  original  discussion. 

E.  s.  a 

ScRANTON,  Pa.,  January,  1888. 


THE  DESIGNING  AND  CONSTRUCTION 


OF 


STORAGE  RESERVOIRS. 


Before  entering  upon  such  considera- 
tions as  affect  the  selection  of  reservoir 
sites  and  their  construction,  a  brief  allu- 
sion to  some  of  the  most  ancient  works 
for  impounding  water  may  not  be  unin- 
teresting. Of  these  the  most  prominent 
examples  are  undoubtedly  to  be  found 
in  Hindostan,  where  the  magnitude  and 
antiquity  of  the  storage  works  cannot  fail 
to  arrest  attention.  These  great  works, 
surpassing  in  their  immensity  what  are 
conventionally  esteemed  to  be  the  won- 
ders of  the  world,  the  production  of  other 
countries  and  nations,  took  their  origin 
in  the  necessities  of  the  people  and  the 


6 


variableness  of  the  climate  of  India,  and 
were,  in  fact,  great  public  works  on  which 
the  welfare  of  the  people  mainly  de- 
pended. The  climate  of  India,  although 
singularly  uniform  in  some  respects  from 
year  to  year,  is  remarkably  variable  as 
regards  the  rain-fall ;  and  in  order  to 
guard  against  the  disasters  of  famine  and 
sickness,  inevitably  attendant  on  a  scanty 
monsoon,  the  native  princes  were  wont 
to  make  such  provisions  as  large  re- 
sources and  an  almost  unlimited  power 
enabled  them,  in  order  to  obviate  the  diffi- 
culty that  they  had  to  contend  with. 

The  rain  records  of  India  for  several 
years  past  show  that  a  scarcity  of  rain  is 
indicated  by  periods  of  about  five  years, 
or  that  every  fifth  or  sixth  year  is  marked 
by  a  scanty  rainfall  over  certain  districts. 
The  recurrence  of  these  periods  is,  of 
course,  not  very  clearly  marked,  but  still 
it  is  sufficiently  so  to  warrant,  with  ap- 
proximate correctness,  the  jDrediction  of 
scarcity  and  famine  ;  and  such  deplorable 
recurrence  is,  as  all  are  aware,  now  reign 
ing  in  India,  and  visiting  with   destruc- 


tion,  by  sickness  and  hunger,  some 
thousands  whose  sole  dependence  is  upon 
a  fair  season  of  rain,  and  the  successful 
maturing  of  their  Httle  crop  of  grain. 

The  natural  expedient  for  guarding 
against  the  recurrence  of  these  periodical 
calamities  was  evidently  to  be  found  in 
husbanding  a  scanty  supply  of  rain-water 
for  the  purpose  of  irrigation,  and  this 
the  people  of  India  appear  to  have  un- 
derstood. They  took  advantage,  in  cer- 
tain districts,  of  every  nook  and  ravine, 
whether  large  or  small,  and  converted 
them  into  storage  reservoirs  by  throwing 
across  banks  of  earth,  or  bunds,  as  they 
are  termed,  producing,  in  certain  dis- 
tricts, such  an  elaborate  and  complete 
system  of  irrigation  as  can  only  be  com- 
pared, for  cost  and  completeness,  to  our 
railway  system  in  England.  Taking  four- 
teen districts  in  the  Madras  Presidency, 
where  tank  irrigation  was  most  generally 
relied  upon,  the  records  of  the  Indian 
Government  show  that  there  are  no  less 
than  43,000  irrigation  reservoirs  now  in 
effective  operation,  and  as  many  as  10,000 


8 

more  that  have  fallen  into  disuse,  making 
a  total  number  of  53,000  storage  works. 
The  average  length  of  embankment  is 
found  to  be  about  half  a  mile,  the  ex- 
treme limit  of  the  series  being  a  dam  of 
the  immense  length  of  30  miles.  This 
ancient  reservoir,  called  the  Poniary 
tank,  is  no  longer  in  use,  the  cost  of 
maintaining  such  a  length  of  bank  in 
adequate  repair  having  probably  been 
found  disproportionate  to  the  advantage 
derived  from  the  supply.  The  work,  em- 
bracing an  area  of  storage  of  between  60 
and  80  sq.  miles,  remains,  however,  as  a 
record  of  what  the  Hindoos  are  capable 
of.  To  quote  a  second  example,  there  is 
the  Veranum  reservoir,  now  in  actual 
operation  as  a  source  of  supply,  and 
yielding  a  net  revenue  of  no  less  than 
£11,450  per  annum.  The  area  of  the 
tank  is  35  sq.  miles,  and  the  storage  is 
effected  by  a  dam  12  miles  in  length. 
In  order  to  bring  the  immensity  of 
this  system  of  storage  works  within 
the  reach  of  statistical  minds,  it  has  been 
calculated  that  the  embankments  contain 


9 


as  much  earth  as  would  serve  to  encircle 
the  globe  with  a  belt  of  6  ft.  in  thickness. 
To  show  that  these  are  not  singular  ex- 
amiDles,  one  other  embankment  of  re- 
markable size  may  be  alluded  to.  This 
embankment,  of  somewhat  smgular  con- 
struction, was  built  on  the  island  of 
Ceylon,  and  bears  testimony  that  the 
Singalesemonarchs  were  not  behind  their 
neighbors  in  public  spirit  or  enterprise. 
The  embankment  was  composed  of  huge 
blocks  of  stone  strongly  cemented  to- 
gether, and  covered  over  with  turf,  a 
solid  barrier  of  15  miles  in  length,  100  ft. 
wide  at  base,  sloping  to  a  tojD  width  of 
40  ft.,  and  extending  across  the  lower 
end  of  a  spacious  valley. 

Thus  it  will  appear  that  the  practice  of 
embanking  across  valleys,  for  the  purpose 
of  retaining  the  surface  water,  has  for 
ages  been  in  operation.  There  is  no 
doubt  that  the  disposal  of  some  of  the 
most  remarkable  works  in  India  is  not 
what  it  might,  with  advantage,  have 
been ;  the  fact  remains,  however,  that 
the  desired  end  was  attained,  and  if  the 


10 


earthworks  were  disproportionately  ex- 
tensive, it  was  a  source  of  satisfaction,  at 
least,  for  the  projectors  to  know  that  they 
cost,  as  a  general  rule  little  or  nothing, 
the  practice  in  those  days  being  to  press 
whatever  labor  was  required,  rendering 
in  return  nominal  wages  or  none  at  all. 

The  two  main  questions  that  it  is  pro- 
posed to  submit  for  consideration  are, 
first,  the  selection  of  a  reservoir  site ; 
and,  secondly,  the  leading  principles  to 
be  observed  in  the  designing  and  con- 
struction of  storage  works. 

The  purpose  or  purposes  for  which  the 
work  may  be  required  will,  of  course,  af- 
fect materially  the  choice  of  a  position,  as 
well  as  the  details  of  the  structure  itself ; 
but  certain  general  principles  are  avail- 
able for  our  guidance  in  every  case,  after 
considering  which,  it  is  proposed  to  dwell 
upon  such  points  as  apply  to  the  special 
purposes  for  which  reservoirs  may  be  con- 
structed. 

The  first  and  most  essential  point  for 
accurate  determination  by  the  engineer  is, 
undoubtedly,  the  amount  of  rainfall,  both 


11 

maximum  and  minimum,  that  may  be  ex- 
pected in  the  district  under  examination  ; 
and,  having  arrived  at  rehable  data  on 
this  point,  the  next  consideration  will 
obviously  be,  what  amount  may  be  made 
available,  due  allowance  having  been 
made  for  evaporation  and  absorption. 
When  we  know  that  the  annual  depth  of 
rainfall  taken  all  over  the  world  varies, 
according  to  the  locality,  between  zero 
and  338  in.  or  28  ft.  deep  (which  ex- 
cessive amount  was  on  one  occasion 
registered  in  the  hill  district  of  Western 
India),  it  will  be  obvious  how  little 
ground  there  will  be  for  assumption,  in 
the  examination  of  any  district  hitherto 
unexplored,  with  regard  to  the  question 
of  its  rainfall.  In  the  examination  of  any 
given  country,  however,  there  are  certain 
phenomena  connected  with  the  rainfall 
that  will  be  found  of  almost  invariable 
acceptation,  and  may  with  advantage  be 
borne  in  mind. 

The  rainfall  will,  as  a  general  rule,  be 
greatest  in  those  districts  that  are  situ- 
ated towards  the  point  from  which   tbe 


12 

prevailing  winds  blow.  If  Great  Britain 
for  instance  be  taken,  the  western  dis- 
tricts will  be  found  the  most  rainy. 
The  very  reverse,  however,  of  this 
phenomena  is  noticed  in  the  neighbor- 
hood of  mountain  ranges.  If  the  wind 
prevails  from  one  side  rather  than  from 
the  other,  it  is  found  that  the  greatest 
rainfall  is  on  the  leeward  side  of  the 
range,  and  the  probable  solution  of  the 
matter  is,  that  the  air,  highly  charged  with 
moisture,  is  carried  up  the  hills  by  the 
wind  until  it  comes  into  a  cold  region 
of  the  atmosphere.  Condensation  of 
the  watery  vapor  immediately  takes 
place,  and  the  result  is  a  fall  of  rain  on 
the  side  of  the  mountain  range  remote 
from  the  prevailing  wind. 

To  this  cause  may  also  be  attributed 
the  fact  that  the  rainfall  is  always  greatest 
in  mountainous  districts,  while  it  by  no 
means  follows  that  elevated  plains  are 
more  abundantly  supplied  with  rain  than 
land  lying  nearer  to  the  sea  level.  The 
principles  are  remarkably  exemplified  in 
the  southern  part  of  the  Bombay  Presi- 


13 

dency,  where  the  author  has  had  occasion 
to  study  the  subject  of  rainfall.  The 
Western  Ghauts  run  parallel  to  the  coast, 
rising  to  a  height  of  4,500  ft.  above  the 
sea,  and  from  the  western  support  of  the 
great  table-land  of  the  Deccan,  the  mean 
elevation  of  which  may  be  taken  at  3,000 
ft.  In  the  rainy  season  the  southwest 
monsoon,  blowing  from  the  sea,  impinges 
agaiust  the  Ghauts,  and  while  passing  on- 
wards to  the  Deccan,  parts  with  its 
moisture  to  the  average  annual  amount 
of  254  in.  On  a  spur  of  mountain  that 
runs  eastward,  the  pluviometers  are  found 
to  register  but  50  in.;  and  about  40 
miles  farther  inland  the  rainfall  is  not 
more  in  some  places  than  15  in.,  which  is 
considerably  less  than  that  registered  in 
the  lower-lying  districts  of  the  Presi- 
dency.* 

*In  regard  to  this  matter,  Mr.  Fanning  says  in  his 
"Water  Supply  Engineering":— "A  study  of  some  of 
our  principal  river  valleys  independently,  reveals 
the  fact  that  their  rainfall  gradually  decreases  from 
their  outlets  to  their  more  elevated  sources."  And 
again:  "  The  oft-made  statement,  'that  rain  falls  most 
abundantly  on  the  high  land,'  is  applicable,  in  the 
United  States,  to  subordinate  water-sheds  only,  and  in 
rare  instances." 


14 


In  civilized  countries  like  our  own, 
much  valuable  information  is  usually- 
available  regarding  the  rainfall,  if  not  ap- 
plying actually  to  the  district  under  ex- 
amination, then  probably  to  some  neigh- 
boring districts,  enjoying  the  same 
physical  characteristics;  but  when  any 
project  of  great  importance  is  in  contem- 
plation, it  will  not  be  sufficient  to  take 
the  returns  of  adjoining  districts  as 
accurate  information  of  the  rainfall  at  the 
exact  locality  fixed  upon  for  the  construc- 
tion of  the  works.  It  will  be  necessary 
to  establish  rain-gauges  at  different  points 
over  the  catchment  basin  of  the  valley 
from  which  it  is  intended  to  obtain  the 
supply ;  and  daily  observations  of  these 
gauges  must  be  taken  for  comparison  with 
a  series  of  simultaneous  observations 
taken  and  recorded  at  the  nearest  station 
at  which  the  rainfall  has  been  regularly 
and  carefully  noted.  It  is  evident  that 
a  comparison  of  the  several  observations 
taken  over  the  area  of  water- shed  with 
those  registered  at  the  permanent  station, 
will  convey  a  just  estimate]  of  the  amount 


15 

of  maximum  and  minimum   rainfall  that 
may  be  relied  upon. 

The  amount  of  rain  falUng  upon  the 
ground  is  not,  however,  the  point  to  be 
determined,  though  it  will  aid  consider- 
ably as  a  guide  to  the  engineer.  A  con- 
siderable quantity  of  all  the  rainfall  is 
either  absorbed  by  the  ground  or  evapor- 
ated before  it  reaches  the  point  at  which 
it  can  be  made  available  for  storage. 
Regarding,  then,  first,  the  question  of 
absorption,  it  must  be  apparent  that  no 
two  districts,  unless  they  are  exactly 
identical  in  soil,  inclination  of  sui'face, 
and  under  similar  circumstances  of  culti- 
vation, can  give  on  examination  the  same 
comparative  result  of  rainfall  and  evapo- 
ration. If  one  district  or  unit  of  area  be 
similar  to  the  other  in  all  respects  but 
the  surface  incUnation,  that  which  has 
the  greatest  slope  will,  as  a  rule,  give  the 
largest  percentage  of  water  available  for 
storage,  because  of  course  there  will  be 
less  time  for  the  rain  to  be  absorbed. 
Again,  the  degree  of  cultivation  will 
materially  afiect  the  result  when  two 
areas,    otherwise    precisely     similar     in 


16 

their  physical  conformation,  come  to  be 
compared  one  with  the  other,  it  being 
evident  that  an  open  and  well-drained 
soil  will  be  more  favorable  to  the  reten- 
tion of  water  falling  upon  it  than  com- 
pact and  impervious  land.  In  every  case 
the  physical  features  of  a  district  will 
each  and  every  one  of  them  force  itself 
on  the  attention,  as  influencing  the  con- 
clusion to  be  arrived  at.  If  any  general 
rule  can  be  applied,  it  may  be  said  that 
the  greater  the  slope  of  the  valley,  the 
more  rapidly  it  will  throw  surface  water 
off ;  the  more  denuded  the  surface  is  of 
soil  of  any  kind,  the  less  will  the  escape 
of  rain-water  be  retarded  ;  and  the  more 
compact  the  rocks  composing  the  geologi- 
cal structure  of  a  district,  the  better  will 
the  circumstances  be  for  impounding 
water.  The  volcanic  rocks  and  those  of 
the  granite  order  will  be  as  favorable  as 
any  that  could  be  desired ;  while,  on  the 
other  hand,  porous  rocks,  such  as  the 
sandstones,  chalk,  etc.,  are  too  absorbent 
to  offer  the  desired  conditions  for  storage. 
It  is  not  here  asserted  that  all  the  water 
absorbed  by  porous  rocks  is  necessarily 


17 


intercepted  from  passing  away  to  con- 
tribute to  storage  supply ;  much  of  it 
may  be  lost  by  evaporation  and  absorp- 
tion by  vegetables,  but  a  considerable 
portion  will  often  be  found  to  contribute 
in  the  form  of  springs,  if  the  disposition 
of  the  strata  be  favorable. 

As  a  further  source  of  loss,  evapora- 
tion from  the  ground  as  well  as  from  the 
surface  of  the  reservoir,  must  be  taken 
into  consideration.  The  circumstances 
attending  the  latter  source  of  loss  will  be 
considered  further  on,  as  this  does  not 
affect  the  question  of  how  much  of  the 
total  rainfall  may  be  made  available.* 

The  question  how  much  water  will  be 
evaporated  at  any  moment  from  the  sur- 
face of  land  is  one  involved  in  consider- 
able difficulty ;  and  so  many  disturbing 
elements  enter  into  the  solution  of  the 
problem,  that  its  accurate  determination 


*It  is  customary,  in  this  country,  when  estimating 
the  available  areas  of  water-sheds,  to  deduct  the 
areas  of  all  lakes,  ponds  and  open  bodies  of  water,  as 
the  evaporation  from  them  is  supposed  to  cancel  the 
rainfall  upon  them. 


18 

may  be  regarded  as  hardly  possible  of 
attainroent.  The  hygrometric  state  of 
the  ground's  surface,  the  aspect  of  the 
sky,  the  amount  of  wind,  and  the  tem- 
perature, will  all,  in  their  degree,  exercise 
a  sensible  influence  on  the  amount  of 
water  tJiat  the  ground  will  give  off  from 
its  surface ;  so  that,  in  fact,  it  is  doubtful 
whether  any  reliable  and  philosophically 
correct  conclusions  can  be  arrived  at.  The 
resultant  facts  from  such  experiments  as 
have  been  carefully  conducted  afford, 
after  all,  the  only  data  for  the  engineer 
to  arrive  at  any  general  conclusion  by; 
and  for  forming  a  rough  estimate  for  the 
probable  available  rainfall  of  a  district, 
the  following  proportions  of  available 
actual  rainfall  may  be  accepted  as  fur- 
nishing general  data  ;  but  they  are  not 
meant  to  obviate  the  necessity  of  a  care- 
ful and  specific  examination  of  the  circum- 
stances likely  to  affect  the  design  of  any 
particular  work : 

steep  surfaces  of  granite,  gneiss,  and  slate  100 

Moorland  and  liill  pasture (JO  to   80 

Flat  cultivated  country 40  to    50 

Chalk Oto     0 


19 


In  order  to  arrive  at  more  specific,  and 
truly  reliable  results,  the  engineer  will 
have  to  make  a  series  of  accurate  observa- 
tions on  the  discharge  of  the  stream  or 
streams  that  carry  away  the  rainfall  of  a 
district ;  and  by  doing  so,  and  at  the  same 
time  comparing  the  result  with  the 
amount  of  rain  registered  by  the  gauges 
— which  should  also,  of  course,  be  kept 
with  accuracy  in  the  locality  under  ex- 
amination— an  approximately  true  esti- 
mate of  the  available  rainfall  wdll  be 
arrived  at. 

If  there  is  time  in  the  preparation  of  a 
project  to  make  the  necessary  examination 
of  a  district,  it  is  evident  that  the  results 
will  speak  for  themselves  ;  and  there  will 
be  no  necessity  to  enter  into  abstract 
speculations  concerning  the  theory  of  the 
influences  affecting  loss  by  either  evapora- 
tion or  absorption. 

In  proportioning  the  size  of  a  storage 
reservoir  to  the  area  of  the  catchment 
basin,  the  engineer  will,  of  course,  in  the 
first  instance  be  guided  by  the  require- 
ments of  the  work.     The  object  of   the 


20 

undertaking  may  be  any  one  of  the  fol- 
lowing : 

To  husband  a  scanty  rainfall. 

To  check  the  injurious  effect  upon  the 
country  by  floods. 

To  add  to  the  discharge  of  a  stream, 
by  preventing  the  escape  of  the  flood 
waters. 

The  amount  of  storage  will  always  be 
part  of  an  engineer's  data  in  designing 
works.  It  will  either  be  his  object  to 
store  the  whole  of  the  water  that  the 
drainage  area  will  afford,  which  will  be 
the  case  in  impounding  water  for  irriga- 
tion, for  example ;  or  a  certain  fixed  de- 
mand, governed  by  the  want  of  a  town  or 
other  requirements,  will  determine  the 
amount  of  the  rainfall  that  it  will  be 
necessary  to  retain  for  supply.  In  Eng- 
land the  demand  for  water  supply  may 
be  reckoned  at  from  150  to  180  days, 
depending  on  the  amount  and  the  con- 
stancy of  the  rainfall ;  as  a  rule,  the  six 
months'  supply  will  be  the  safest  to  adopt. 
The  following  table  extracted  from  Mr. 
Beardmore's  work,  shows  the  proportions 


21 


that  have  been  observed  in  designing 
some  of  the  best  constructed  reservoirs. 
(See  Table  A.)* 

From  this  it  appears  that  the  propor- 
tion between  the  amount  stored  and  the 
total  rainfall  varies  between  one-half  and 
one-fourth. 


*There  are  a  number  of  discrepancies  in  this  table, 
but  as  the  figures  in  the  different  columns  are  func- 
tions of  each  other,  it  would  be  very  diificult,  if  not 
impossible,  to  say  where  the  errors  lie. 


22 


Table  A. 


I 


Locality. 

Height 
above  Sea. 

1 

< 

1 

Bann  Reservoirs,  1837-38.... 
Greenock.  1827-28,  flat  moor. 

Bute,  1820 

Glencorse,  Pentland  Hills.. 

Belmont,  1843,  moorland 

"          1844, 

ft.          ft. 

400    to    2800 
512    to    1000 
200    to      350 
734    to    1600 
650    to    1600 

sq.  m. 

5.15 
7.88 
7.80 
6.00 
2.81 

"          1845,         " 

1846,         " 

Rivington  Pike,  1847 

Longendale            "    

Swineshaw            "     ........ 

Turton  and  Entwistle,  1836 
1837 

800   to    1545 
500    to    1800 
600    to    18U0 
500    to    1300 

16.25 
'"'3!i8' 

Bolton  Waterworks 

800   to    1600 
800 

80 

Ashton         "             1844 

.59 

23 


Table  A — Continued. 


o 

<D 

;_ 

br-- 

5 

^ 

'd 

cu 

ta 

o 

1^ 

Si 

11 

c3 

^3 

O    o 

2  « 

?S 

%< 

1<  =2 

g"^ 

m 

a, 

2<1 

11 

O 

OJ 

d 

Sh    O 

H 

P 

« 

K 

Ph"" 

c.  ft. 

C.    ft. 

ir. 

. 

c.  ft. in. 

perm. 

perm. 

in. 

in. 

mill. 

1092.6 

210.2 

48.0 

72.0 

56.0 

* 

1416.6 

197.7 

41.0 

60.0 

38.0 

2-5 

819.0 

105.0 

23.9 

45.4 

600.0 

100.0 

22.3 

37.0 

7.66 

3.10 

630.4 

224.3 

50.7 

63.4 

26.8 

\ 

412.8 

146.4 
181.9 
146.3 

33.3 
41.2 
33.2 

50  0 
55.0 

49.8 

511.2 

411.3 

2880.0 

176.7 

40.0 
49.5 
37.0 
41.0 

55.5 
55.5 
49.3 
46.2 

29.6 

i 

.... 

576.7 

181.8 

31.43 

^ 

548.2 

172.3 

125.2 

39.0 
32.7 

48.2 

100.2 

25.6 

1-8 

40.7 

65.5 

15.5 

40.0 

21.0 

4-7 

24 


The  rule  suggested  by  Professor  Ran- 
king "  for  estimating  the  available  capa- 
city required  in  a  store  reservoir,  that 
founded  upon  taking  into  account  the 
supply  as  well  as  the  demand,"  is  prob- 
ably the  best  that  can  be  adopted  in  de- 
signing waterworks  for  the  supply  of  a 
town ;  "  for  example,  180  days  of  the  ex- 
cess of  the  daily  demand  above  the  least 
daily  supply,  as  ascertained  by  gauging 
and  computation  in  the  manner  above 
described."  In  order  that  a  reservoir  of 
the  capacity  "  prescribed  by  the  preced- 
ing rule  may  be  efficient,  it  is  essential 
that  the  least  available  annual  rainfall  of 
the  gathering  grounds  should  be  suffici- 
ent to  supply  a  year's  demand  for  water." 
In  calculating  the  capacity  of  a  storage 
reservoir,  the  consideration  of  the  sur- 
face evaporation  must  not  be  disregarded, 
especially  when  the  works  are  designed 
for  tropical  or  very  dry  climates.  The 
amount  of  loss  will  in  some  cases  be  very 
considerable,  for  whatever  depth  of  water 
be  assumed  to  pass  away  into  the  air,  it 
must  be  regarded  as  extending  over   the 


25 


whole  surface  of  the  reservoir  ;  or,  in  fact, 
the  cubic  quantity  will  be  equal  to  the 
product  of  the  depth  evaporated  away 
and  the  mean  surface  area  of  the  reservoir 
as  the  water  rises  or  falls  throughout  the 
year.  Some  have  gone  the  length  of  as- 
serting that  the  amount  of  evaporation 
from  the  surface  of  large  and  deep  bodies 
of  water  is  probably  nothing  at  all,  or  at 
any  rate,  not  worthy  of  consideration ; 
whilst  others  assume  a  much  larger 
amount  of  loss  than  appears  to  be  sup- 
ported by  observation.  The  following 
extract  from  the  article  "  Physical  Geog- 
raphy," published  by  the  Society  for 
Promoting  Useful  Knowledge,  expresses 
intelligibly  the  conditions  that  tend  to 
promote  evaporation : 

"  Other  things  being  equal,  evaporation 
is  the  more  abundant  the  greater  the 
warmth  of  the  air  above  that  of  the  evap- 
orating body,  and  least  of  all  when  their 
temperature  is  the  same.  Neither  does 
much  take  place  whenever  the  atmosphere 
is  more  than  15  deg.  colder  than  the  sur- 
face upon  which  it  acts.     Winds  power- 


26 


fully  promote  evaporation,  because  they 
bring  the  air  into  continual  as  well  as  in- 
to closer  and  more  violent  contact  with 
the  surface  acted  upon,  and  also,  in  the 
case  of  liquids,  increase  by  the  agitation 
which  they  occasion,  the  number  of  points 
of  contact  between  the  atmosphere  and 
the  liquid. 

"In  the  temperate  zone,  with  a  mean 
temperature  of  52 j^  deg.,  the  annual  evap- 
oration has  been  found  to  be  between 
36  in.  and  37  in.  At  Cumana,  on  the 
coast  of  South  America  (N.  lat.  10^),  with 
a  mean  temperature  of  81.86  deg.,  it  was 
ascertained  to  be  more  than  100  in.  in  the 
course  of  the  year  ;  at  Guadaloupe,  in  the 
West  Indies,  it  has  been  observed  to 
amount  to  97  in.  The  degree  of  evapora- 
tion very  much  depends  upon  the  differ- 
ence between  the  quantitj'  of  vapor  which 
the  surrounding  air  is  able  to  contain 
when  saturated  and  the  quantity  which  it 
actually  contains.  M.  Humboldt  found 
that  in  the  torrid  zone  the  quantity  of 
vapor  contained  in  the  air  is  much  nearer 
to  the  point  of  saturation  than    in    the 


27 

temperate  zone.  The  evaporation  within 
the  tropics,  and  in  hot  weather  in  temper- 
ate zones,  is  on  this  account  less  than 
might  have  been  supj^osed  from  the  in- 
crease of  temperature." 

Thus  it  appears  that  evaporation,  under 
highly  favorable  conditions,  may  take 
place  to  the  extent  of  9ft.  in  depth — an 
allowance  that  will  demand  careful  con- 
sideration in  designing  storage  works. 
In  India,  where,  from  the  extreme  dryness 
of  the  atmosphere,  the  evaporation  is 
found  to  be  considerable,  the  usual  al- 
lowance made  by  engineers  for  evapora- 
tion from  the  surface  of  storage  reservoirs 
is  at  the  rate  of  ^  in.  of  depth  per  diem 
for  eight  months  in  the  year.  Regarding 
the  results  that  have  been  arrived  at  in 
Bombay,  this  allowance  would  appear  to 
be  about  double  what  is  necessary,  for  the 
observations  extending  over  five  years 
give  a  mean  daily  evaporation  of  less  than 
^  in.  In  Bombay,  however,  the  atmo- 
sphere is  much  more  humid  than  that  ex- 
perienced on  the  great  tableland  of  the 
Decaan ;  and  in  Madras,  where  reservoirs 


28 


are  the  specialty,  it  is  probable  that  the 
actual  loss  is  not  far  from  being  a  mean 
between  the  two  fractions.  In  Great 
Britain  the  mean  daily  evaporation  is 
found  to  average  less  than  the  tenth  of 
an  inch.* 

In  estimating  the  quantity  of  storage 
water  that  will  result  from  the  drainage 
of  any  particular  district,  it  will  be  essen- 
tial to  consider  carefully  the  geological 
disposition  of  the  strata  characterizing 
the  locality  in  which  it  is  contemplated 
to  establish  the  works.  This,  although  a 
matter  that  may  influence  the  effective- 
ness of  an  undertaking  to  the  extent  of 
success  or  failure,  will  appear  to  thfe  pure- 
ly practical  man  to  imply  a  degree  of  re- 
finement that  is  uncalled  for.  There  will 
be  no  difficulty,  however,  in  showing  that 
the  geological  conformation  of  a  district 
may  be  such  as,  on  the  one  hand,  to  ma- 
terially contribute  to  the  efficiency  of  a 

*See  an  interesting  paper  upon  Evaporation,  by  Mr. 
Desmond  FitzGerald,  M.  Am.  Soc.  C.  E.,  contained  in 
the  Transactions  of  the  American  Society  of  Civil 
Engineers,  for  Sept.,  1886. 


29 


storage  reservoir,  or  on  the  other  to  prove 
so  defective  that  no  engineering  skill  or 
pecuniary  outlay    could   remedy  it.      A 
condition  of  geological  structure,  perhaps 
the  most  favorable  that  could  be  imagined, 
is  that  shown  in  Fig.  1.      This  diagram 
represents  a  geological    section  taken  at 
right  angles,  or  nearly  so,  to  the  axis  of 
the  valley  that  it  is  proposed  to  convert  to 
the  purpose  of  storage.     This  somewhat 
peculiar  structure  is  what  is  geologically 
termed  synclinal,  the  beds  inclining  away 
from  the  axis  of  the  valley,  and  is  the  re- 
sult of  an  upheaving  force  having  taken 
place  underneath  the  points  of   greatest 
elevation.      Subsequent   to  the  upheaval 
and    consequent    displacement    of     the 
strata,    the   process    of   denudation   has 
taken  place,  cutting  the  upper  beds,  and 
leaving  the  outcrop  exposed,  not  only  in- 
side   the    basin,    but    in   the   adjoining 
valleys  at  O  and  O.     Now,  it  is   evident 
that  if  the  highest  ridges  bounding  the 
valley  be  taken  to  mark  the  line  of  water- 
shed, and  therefore  limiting  the  area  of 
the  catchment  basin,  it  is  possible  that 


r 


■■) 


31 


the  estimate  of  the  umount  of  supply  may 
be  found  far  short  of  what  the  district  will 
yield.     A  certain  proportion  of  rain  fall- 
ing upon  the  outcrop  at  the  points  O  O 
will  be  absorbed  by  such  of  the  strata  as 
are  porous,  and  the   water,  percolating 
through  the  bedding,  till  an  impervious 
stratum  is  met  with,  will   find  its  way 
down  the  course  of  the  stratification,  till 
it  ultimately  reaches  the  reservoir  in  the 
form  of  springs,  and  contributes  more  or 
less  to   the  maintenance  of   the  supply. 
The  converse  of  this  condition  of  things 
will  be  readily  understood  by  reference 
to  Fig.  2.     It  also  represents  a  section 
taken  directly  across    the   valley  of   the 
proposed  reservoir.     Here  the  strata  of 
the  earth's  crust  incline  against  each  other 
consequent  upon   some  disturbing  force 
having  taken  place  to   elevate  them,  and 
are  said  to  be  anticlinal  to  the  axis  of  the 
valley.     In  order  to  account  for  the  for- 
mation of  a  valley  on  the  summit  of  the 
ridge,  that  at  first  was  thrown  up,  it  is  to 
be  understood  that  the  upper  beds   suf- 
fered fracture  in  the  process  of  upheaval 


32 


33 

and  subsequently  were  exposed  to  de- 
nudation. These  valleys  of  elevation 
are  evidently  not  to  be  desired  as  situa- 
tions for  the  establishment  of  storage 
reservoirs.  The  area  of  the  gathering 
grounds  will  be  much  more  limited  than 
the  extent  of  the  water-shed  would  ap- 
pear to  indicate ;  and  cannot  safely  be 
relied  upon  to  give  an  estimate  of  the 
quantity  of  water  that  the  valley  will 
afford.  A  certain  amount  of  water  will 
undoubtedly  pass  over  the  surface  in 
times  of  heavy  and  continued  rain,  before 
it  can  be  absorbed;  but  there  is  no 
doubt  that  of  all  the  water  absorbed  by 
the  ground,  by  far  the  greater  portion 
will  follow  the  inclination  of  the  strata, 
and  come  out  as  springs  in  the  adjoining 
valleys. 

Fig.  3  shows  a  geological  section  that 
combines  in  it  favorable  and  unfavorable 
conditions  for  the  storage  of  water.  On 
one  side  the  outcrops  of  the  strata  are 
found  to  extend  beyond  the  highest  point 
of  water-shed  line,  whilst  on  the  other 
side  the  strata  incline  away,  producing 


34 

such  a  condition  as  would  favor  the 
escape  from  the  valley  of  the  water  ab- 
sorbed. 

Certain  rules  are  in  general  use  for 
estimating  the  quantity  of  the  total  rain- 
fall that  will  be  lost  by  absorption  and 
evaporation,  with  a  view  to  determining 
the  proper  proportion  to  be  observed  be- 
tween the  reservoir  and  the  area  of  the 
catchment  basin.  Two-thirds  of  the 
whole  fall  is  sometimes  taken  to  represent 
the  loss  that  may  be  expected  from  the 
drainage  of  any  district;  in  general  terms, 
one-third  being  assumed  as  the  amount 
that  may  actually  be  intercepted  for  util- 
ization. Some  authors  leave  a  much 
smaller  margin,  and  state  that  fully  two- 
thirds  of  the  total  rainfall  may  fairly  be 
taken  as  available  for  storage.  This  is  a 
large  discrepancy  when  the  aj^plication 
of  the  rules  is  taken  to  be  general ;  but 
when  the  statements  are  applied  to  sepa- 
rate districts  and  different  countries,  there 
is  nothing  irreconcilable  in  them.  Gen- 
eral rules  are  undoubtedly  of  much  value 
if    they  be  received    with  quahficalion, 


35 


and  are  not  adopted  as  of  absolutely  uni- 
versal application.  They  cannot,  how- 
ever, with  safety  be  substituted  for  speci- 
fic investigations,  when  so  much  depends 
on  starting  with  accurate  data. 

RESERVOIR    SITES. 

The  special  requirements  of  each  partic- 
ular case  will,  as  a  general  rule,  go  far 
towards  determining  the  selection  of  a  site 
for  the  establishment  of  storage  works. 
Assuming,  however,  that  there  is  a  consid- 
erable extent  of  country  situated  ad- 
vantageously in  relative  position  to  the 
locality  at  which  it  is  proposed  to  utilize 
the  water,  and  that  there  is  a  choice  of 
ground,  the  point  to  be  considered  chiefly 
will  be  the  natural  lie  of  the  country. 
To  throw  an  embankment  across  a  valley 
at  any  point  without  due  regard  to  the 
.configuration  of  the  ground  would  most 
probably  result  in  an  expensive  and  ill- 
designed  scheme  ;  for  under  such  circum- 
stances the  cost  of  the  dam  would  bear 
a  very  large  proportion  to  the  quantity 
of  water  stored.  It  will  rarely  happen 
that,  in  the  examination  of  the  resources 


37 


of  any  particular  piece  of  country,  some 
special  features  will  not  present  them- 
selves, favorable  to  the  situation  of  stor- 
age works.  The  most  advantageous  dis- 
position of  the  ground  will  be  when  two 
spurs  of  high  land  approach  each  other, 
forming  a  narrow  outlet  for  the  stream, 
and  leaving  a  wide  space  above  them  in  the 
valley  for  storage.  Such  a  configuration 
is  not  uncommonly  met  with  at  the  junc- 
tion of  two  streams,  as  shown  in  Fig.  4. 
This  is  merely  a  sketch  from  memory,  by 
the  author,  of  a  reservoir  that  he  had  de- 
signed in  India  for  purposes  of  irrigation; 
and  it  will  be  evident  that  the  dis^Dosition 
of  the  ground  was  singularly  favorable 
in  every  respect  for  the  construction  of  a 
large  storage  work.  The  area  of  the 
reservoir,  as  designed,  was  about  three 
square  miles,  and  the  maximum  depth  90 
ft.,  the  area  of  the  catchment  basin  being 
about  60  square  miles.  Such  favorable 
situations  for  storage  are  of  somewhat  rare 
occurrence  ;  for  when  the  contour  of  the 
land  is  what  is  desirable,  it  may  be  that 
the  area  of  water-shed  is  not  adequate,  or 


38 


39 


possibly  the  geological  condition  of  the 
ground  may  be  unfavorable,  or  the  ma- 
terials for  the  construction  of  a  sound 
bank  are  not  available.  In  examining 
large  tracts  of  country  in  India,  with  a 
view  to  the  establishment  of  irrigation 
reservoirs,  the  author  found  that  more 
reliance  was  to  be  placed  on  a  careful  ex- 
amination of  the  map  in  the  first  instance, 
than  on  the  common  plan  of  making  per- 
sonal explorations  of  the  country.  A 
good  map  will  show  at  a  glance,  especi- 
ally if  the  hill-shading  has  been  carefully 
engraved,  the  points  at  which  the  supply 
will  be  found  sufficient  to  justify  the  un- 
dertaking ;  and  will  probably  furnish  a 
pretty  true  indication  of  sites  at  which 
embankments  may  be  advantageously 
constructed. 

In  tropical  climates,  where  the  rainfall 
is  in  places  very  scanty,  and  where  the 
land  is  not  of  great  value,  it  not  unfre- 
quently  happens  that  such  situations 
prove  available  for  the  estabhshment  of 
large  storage  works  as  would  not  under 
any  circumstances  be  made  available  in 


40 


England.  These  sites  are  to  be  found, 
not  at  the  head  of  a  valley,  but  at  some 
considerable  distance  down  the  course 
of  a  stream,  where,  the  general  inclination 
of  the  country  being  slight,  a  low  em- 
bankment serves  to  store  a  very  large 
area  of  water.  The  apparent  disadvant- 
ages of  such  a  site  for  storage  are  the 
large  area  of  land  swamped  and  lost  to 
the  cultivator  and  to  Government,  and 
the  great  surface  exposed  to  evaporation 
under  a  tropical  sun  and  the  influence  of 
a  dry  wind.  In  India,  the  first  objection 
is  one  of  comparatively  little  moment, 
considering  that  in  those  districts  where 
irrigation  is  most  required  the  value  of 
land  is  very  trifling.  From  Is.  to  2s.  is 
about  an  average  rent  per  acre,  where 
land  is  under  dry  crops ;  but  when  water 
is  available,  the  cultivators  can,  with 
profit,  afford  to  pay  30s.  per  acre.  It  is 
therefore  evident  that,  so  far  as  Govern- 
ment is  concerned,  there  is  no  sacrifice 
in  the  matter,  but,  on  the  contrary,  an 
unspeakable  benefit  is  conferred  on  those 
land  owners  who  hold  farms   below  the 


41 


reservoir ;  and  an  ample  supply  of  water 
is  stored  in  the  dryest  seasons  to  mature 
those  crops  whose  failure  almost  inevit- 
ably reduces  the  people  to  the  verge  of 
starvation.  The  evaporation  from  these 
lakes  is,  beyond  question,  a  source  of 
very  considerable  loss,  and  one  that  ad- 
mits of  no  possible  abatement.  Esti- 
mated as  above,  at  about  half  an  inch 
vertical  for  eight  months  of  the  year,  the 
loss  frequently  amounts  to  one-third  of 
the  whole  body  of  water  stored.  As  a 
set-off  against  this  and  other  objections, 
the  facilities  for  constructing  these 
reservoirs  of  great  extent,  are  consider- 
able. In  the  first  place,  the  embank- 
ments, being  very  low,  are  rapidly  and 
cheaply  constructed  by  native  workmen ; 
and  when  finished,  the  head  of  water  even 
at  the  deepest  point  is  not  sufficient  to 
try  the  work  to  any  great  extent.  Fur- 
ther, the  greater  the  extent  of  the  reser- 
voir, the  less  inconvenience  is  experienced 
from  silting.  The  streams,  owing  to  the 
suddenness  of  the  rainfall,  come  down 
heavily  charged  with  earth  in  suspension, 


42 


the  mass  of  which  is  deposited  Hke  a 
miniature  delta  at  the  influx  of  the  reser- 
voir, instead  of  passing  on  and  resting 
near  the  embankment,  as  invariably 
occurs  in  reservoirs  of  small  extent. 
The  immense  consumption  of  water 
necessary  to  confer  any  appreciable  bene- 
fit by  irrigation  is  of  itself  the  strongest 
argument  in  favor  of  these  broad  and 
shallow  reservoirs  ;  for  it  is  not  possible 
to  find  in  the  upper  part  of  a  valley  such 
sites  as  would  store  the  requisite  quan- 
tity of  water  without  an  embankment  of 
excessive  dimensions  ;  and  moreover,  the 
catchment  area  in  such  situations  is  not 
usually  sufficient  to  serve,  with  a  scanty 
rainfall,  for  the  supply  of  a  very  large  re- 
servoir. It  is  not,  of  course,  maintained 
that  this  mode  of  storing  water  is  by  any 
means  applicable  in  England,  for  the  cir- 
cumstances and  requirements  in  each  case 
are  wholly  dissimilar. 

SUPPLY. 

The  reservoir  site  being  supposed 
everything  that  could  be  desired,  as  re- 
gards the  disposition  of  the  ground,  the 


43 


supply  will  next  engage  attention  as  a 
matter  of  course.  Assuming  that  the 
gathering  grounds  are  sufficiently  exten- 
sive, it  is  presumed  that  the  reservoir 
will  be  constructed  to  contain  sufficient 
water  to  meet  the  maximum  demand, 
whatever  that  may  be  calculated  at ;  and 
in  order  to  determine  with  accuracy  what 
capacity  the  reservoir  will  have  with  dif- 
ferent heights  of  embankment,  it  will  be 
necessary  to  carry  out  certain  leveling 
operations  over  the  ground.  The  least 
elaborate  manner  of  proceeding  will  be 
to  run  a  series  of  cross-levels  through 
the  valley,  referring  all  to  the  same  datum, 
and  by  comparing  these  levels  to  ascer- 
tain what  the  average  depth  will  be  for  a 
given  height  of  bank.  Having  decided 
the  height  of  the  water-level,  the  next 
operation  will  be  to  contour  round  the 
basin,  and  to  survey  the  boundary-line. 
In  this  way  may  be  acquired  sufficient 
knowledge  as  to  the  storage  capacity, 
to  justify  the  procedure  with  the  work. 
When  the  execution  of  the  project  has 
been  determined  upon,  it  will  be  advis- 


44 

able  to  make  a  more  accurate  survey  of 
the  bed  of  the  valley,  and  this  can  best 
be  done  by  covering  the  whole  plan  with 
a  series  of  contour  lines  at  a  vertical  dis- 
tance from  each  other  of  about  5  ft.  This 
kind  of  survey  will  be  of  lasting  value  to 
the  engineer,  for  it  will  enable  him  to 
calculate  what  quantity  of  water  the 
reservoir  will  contain  at  each  foot  of 
depth ;  and,  consequently,  he  will  know, 
from  a  mere  inspection  of  the  gauge  in 
the  reservoir,  how  much  water  he  has  at 
his  disposal  for  service. 

It  has  been  assumed  that  the  gathering 
grounds  are  sufficient  to  maintain  the  re- 
quisite suj^ply  in  the  reservoir ;  but  it  may 
be  well  to  pause  and  inquire  what  extent 
of  water-shed  will  be  sufficient  to  furnish 
a  given  supply,  and  what  method  may  be 
adopted  for  supplementing  an  insufficient 
drainage  area.  It  has  before  been  re- 
marked that  the  only  reliable  information, 
when  there  is  any  question  as  to  the 
sufficiency  of  the  rainfall  or  the  area  of 
the  catchment  basin,  can  be  derived  from 
careful  gaugings  of  the  stream  or  streams 


45 

that  may  be  depended  upon  to  contribute 
to  the  supply.  If  the  catchment  area  is 
very  large  as  compared  to  the  capacity  of 
the  reservoir,  a  mere  inspection  of  the  map 
and  an  exploration  of  the  ground  will  gen- 
erally be  conclusive  as  to  the  sufficiency 
of  the  supply  for  storage.  Should  there 
not  be  such  conclusive  evidence  on  this 
point,  it  must  be  determined  by  measur- 
ing the  quantity  of  water  that  absolutely 
flows  off  the  ground,  at  the  same  time 
gauging  the  rainfall.  This  latter  pre- 
caution would  appear  unnecessary,  but  in 
truth  it  is  of  great  value,  for  it  will  fur- 
nish by  comparison  with  the  rainfall 
registers  that  have  been  kept  through  the 
same  year,  and  a  series  of  previous  years, 
evidence  as  to  the  amount  of  available 
rainfall  that  may  be  expected  during 
terms  of  comparative  drought.  If  the 
supply  of  a  town  with  water  be  the 
desideratum,  the  rule  to  be  rigidly 
observed  is  that  of  making  a  minimum 
supply  meet  the  maximum  demand,  and 
therefore  it  is  of  the  highest  importance 
to  determine  beyond  any  doubt,  what  the 


46 


minimum  yield   of    a    catchment    basin 
will  be. 

As  a  mode  of  supplementing  an  insuf- 
ficiently large  drainage  area,  catchment 
drains  or  feeders  have  frequently  ren- 
dered good  service.  These  are  cuts  that 
are  carried  outside  the  water-shed  line 
to  arrest  the  surface  drainage  and  catch 
the  contributions  of  small  streams,  and 
conduct  the  water  into  the  reservoir. 
The  greater  the  area  enclosed  between 
catchment  drains  and  the  water- shed 
line,  the  more  valuable  will  they  be  as 
aids  to  the  supply  of  the  reservoir.  They 
of  course  virtually  extend  the  area  of  the 
catchment,  adding  so  many  square  miles 
or  acres  to  the  rainfall. 

DESIGNING    OF    WORKS. 

Knowing  the  exact  requirement  of  a 
given  population,  or  rather  having  fixed, 
after  every  consideration,  the  daily  con- 
sumption of  every  individual  that  it  is 
proposed  to  supply,  there  will  be  no  dif- 
ficulty whatever  in  proportioning  the 
reservoir  to  the  demand  upon  it.  It  is 
sometimes  necessary,  however,  to  provide 


1 


47 


reservoirs  for  the  purpose  of  preventing 
damage  to  the  country  by  floods,  and  in 
this  way  the  inconvenience  and  injury 
naturally  consequent  upon  very  sudden 
and  excessive  falls  of  rain  may  be  to  a 
great  extent  obviated.  The  duty  of  the 
reservoir  will  be  to  arrest  all  water  in 
excess  of  what  the  stream  can  carry 
within  its  banks,  and  to  dispose  of  this 
excess  water,  so  to  speak,  in  detail,  after 
the  excessive  rainfall  has  become  mod- 
erated. A  comparison  of  a  stream's  dis- 
charge, taken  at  highest  floods,  with  the 
quantity  that  it  can  carry  without  over- 
flowing its  banks,  will  show  the  excess 
that  has  to  be  retained  by  the  reservoir ; 
and  these  data  can  only  be  arrived  at 
through  a  carefully  kept  record  of  the 
extent  of  the  floods  and  of  their  duration. 
The  maximum  flood  in  this  consideration 
will  not  be  that  which  rises  to  the 
greatest  height  for  a  short  time,  but  will 
be^the  product  of  the  excess  above  what 
the-river  can  discharge  by  the  length  of 
time  the  flood  lasts ;  which  will,  in  fact, 
be  the  necessary  capacity  of  the  reservoir. 


48 

The  table  given  on  a  preceding  page 
will  afford  an  interesting  study  when 
compared  with  the  following  table,  ex- 
tracted in  part  from  the  same  work.  The 
first  gives  a  comparative  view  of  the 
volume  of  water  gauged  and  stored  in 
small  hill  districts,  the  last  column  indi- 
cating the  proportion  of  the  total  avail- 
able rainfall  to  the  amount  actually 
intercepted  for  storage.  The  following 
table  shows  the  ordinary  summer  dis- 
charge of  various  rivers,  streams,  and 
springs,  as  unaffected  by  immediate  rain. 
(See  Table  B.)* 

Where  the  reservoir  is  designed  to 
check  the  injurious  effects  of  floods,  the 
proportion  of  the  storage  to  the  rainfall 
will,  in  most  cases,  be  much  smaller  than 
what  would  be  necessary  to  provide  for 
the  better  part  of  a  whole  year's  fall  of 
rain,  for  it  is  not  probable  that  the  max- 
imum known  flood  can  ever  exceed  the 
amount  that  it  would  be  necessary  to 
store  for  economic  purposes. 


♦This  table  also  contains  several  evident  inaccura- 
cies ;  where  the  point  of  error  seemed  apparent,  cor- 
rection has  been  made  in  the  present  edition. 


49 


Table  B. 


RlTERg. 

Height 
above  Sea. 

Thames  at  Staines,  chalk,  greensand 
Oxford  clay,  oolites,  etc 

Severn    at  Stonebench,  Silurian 

Loddon  (February,  1850),  greensand. 

Nene,      at     Peterborough,     oolites, 
Oxford  clav  and  lias         

Valley  Hill, 
ft.       ft. 

40  to    700 
400  to  2600 
110  to    700 

10  to    600 

Mimram,  at  Panshanger,  chalk 

Lee,    at  Lee  Bridge,  chalk  -(Rennie, 

April,  1796) 

Wandle,  below  Carshalton,  chalk. . . . 
Medway,    dryest     seasons    (Rennie, 

1787),  clay 

200  to    500 

30  to    600 
70  to    3cO 

Ditto,  ordinary  summer  run  (Rennie, 

1787) 

Verulam,  at  Bushey  Hall,  chalk 

Gade,  at  Hunton  Bridge,  chalk 

Plym,  at  Sheepstor,  granite 

Woodhead  Tunnel,  millstoue,  grit. . . . 
Glencorse  Bum 

150  to    500 
150  to    500 
800  to  1500 

1000 
750  to  1600 

50 


Table  B. — Continued. 


'•o 

& 

1 

1 

«f-i 

m 

a 

;3 

c3 

§)^. 

M 

*^S 

2 
< 

1^ 

'Si    gj 

""i 

bog 
•2  2 

^1 

? 

§dI 

^< 

'z< 

^ 

"6 

s 

^^ 

a 

>-H 

A 

?i  ft 

^  0. 

'^ 

s 

S 

ft 

5 

;-i 

o 

0 

0 

Q 

H 

s 

K 

H 

sq. 
miles. 

c.    ft. 
per.  min. 

C.    ft. 

per  min. 

in. 

in. 

3086 

40.000 

12  98 

2.93 

24.5 

3900 

33,111 

8.49 

1.98 

221.8 

3  000 

13.53 

3.01 

'25!4 

620.0 

5,000 

8.45 

1.88 

23.1 

50.0 

1,200 

24.3 

5.5 

26.6 

570.0 

8  880 

15.58 

3.53 

41.0 

1,800 

43.9 

9.93 

"24!0 

481  5 

2,209 

4.59 

1.04 



481.5 

2.520 

5.23 

1.19 

120.8 

1,800 

14  9 

3.37 

69.5 

2,500 

36.2 

8  19 

7.6 

500 
139 

71  4 

15.10 

'45;6 

46  0 

6*.6' 

130 

"21.6"' 

""--' 

37.4 

51 


PROPORTIONS    OF    BANK. 

The  proper  proportion  to  be  given  to  an 
embankment  for  the  support  of  water  is  a 
question  that  appears  to  admit  of  a  good 
deal  of  difference  of  opinion,  some  de- 
signers taking  one  view,  some  another,  of 
the  proper  theory  that  is  to  determine 
the  dimensions  of  a  bank.  Some  few, 
with  whom  the  author  cannot  agree  on 
this  point,  maintain  that  a  bank  ought 
to  be  designed  with  strict  reference 
to  its  theoretical  power  of  resisting 
hydrostatic  pressure,  or  the  effort  of  the 
water  to  displace  it.  Kegarding  the 
question  in  its  abstract  form,  it  will  be 
evident  that  any  structure  intended  to 
sustain  the  pressure  of  water  may  be  sup- 
posed to  fail  in  one  of  two  ways  — either, 
in  the  first  place,  by  yielding  to  the 
horizontal  pressure  of  the  water  and  over- 
turning, or  by  progressive  motion,  i.  e., 
sliding  on  its  base.  In  considering  the 
first  theory,  that  of  resistance  of  over- 
turning, the  easiest  method  of  examining 
the  question  will  be  to  take  a  simple  ex- 
ample of  a  vertical  rectangular  wall,  and 


52 

ascertain  what  power  it  exercises  to  resist 
the  pressure  of  water.  The  pressure  of 
water  upon  any  plane  surface  immersed 
is  known  to  be  equal  to  the  area  of  that 
surface,  multiplied  by  the  depth  of  its 
center  of  gravity  below  the  level  of  the 
water.  Generally  speaking,  the  unit 
adopted  in  calculation  is  a  foot ;  and  the 

Fig.  5. 


unit  of  water  being  taken  at  a  cubic  foot, 
weighing  62.5  lbs,  the  resulting  product 
from    the    multiplication   of    the    three 


53 


quantities  will  give  the  pressure  in 
pounds  on  the  surface  immersed.  Let  it 
be  supposed,  for  simplicity,  that  water  to 
the  depth  of  10  ft.  has  to  be  sustained  by 
a  vertical  rectangular  wall,  as  in  Fig.  5. 
It  is  usual  to  take  but  1  ft.  length  of  the 
wall  for  the  calculation,  though  it  will  not 
affect  the  result  whether  1  ft.  or  100  ft. 
be  the  length  assumed.  We  then  have 
the  surface  under  pressure  =  10  sq.  ft., 
the  depth  of  the  center  of  gravity  =  5  ft., 
and  the  weight  of  a  cubic  foot  =  62.5  lbs., 
the  product  of  which  quantities  gives  us 
3,125  lbs.  pressure  on  1  ft.  length  of  the 
wall.  But  this  pressure  is  not  the  whole 
of  the  force  that  the  wall  has  to  resist ; 
the  leverage  that  it  exerts  must  also  be 
taken  into  account.  In  the  example 
under  consideration — viz.,  that  of  a  verti- 
cal plane  with  one  of  its  sides  coinciding 
with  the  surface  of  the  water,  as  in  Fig. 
5 — the  whole  of  the  pressure  is  so  dis- 
tributed as  to  be  equal  to  a  single,  force 
acting  at  a  point  one-third  of  the  depth 
from  the  bottom.  Thus,  the  total  force 
to  be  resisted  by  the  wall  is  3,125  X  3.33 


54 


=  10,406,  which  is  the  moment  tending 
to  overturn  the  wall. 

It  is  evident  that  a  certain  weight  of 
the  wall  must  be  opposed  to  this  over- 
turning force ;  and  as  the  height  of  the 
wall  and  the  length  are  determined 
quantities,  the  thickness  alone  remains 
for  adjustment.  But  as  a  rectangular 
wall  in  upsetting  is  considered  to  turn 
upon  a  single  point,  F,  in  the  Figure — 
viz.,  the  outer  line  of  the  wall — there  will 
be  a  certain  amount  of  leverage  to  assist 
the  wall  in  resisting  the  pressure  of  the 
water.  This  leverage  is  the  horizontal 
distance  of  the  center  of  gravity  of  the 
wall  from  the  turning  point  F,  and  when 
the  structure  is  rectangular  and  vertical, 
it  is  equal  to  half  the  thickness.  The 
amount  of  the  wall's  resistance  will  then 
be  equal  to  the  number  of  cubic  feet  in 
one  foot  of  its  length  multiplied  by  the 
weight  of  a  single  cubic  foot  of  masonry 
and  by  half  the  thickness  of  the  wall. 
Taking  w  =  weight  of  a  cubic  foot  of 
water  =  62.5  lbs.  w'  =  weight  of  a  cubic 
foot  of  brickwork,  say  112  lbs.,  x  =  thick- 


55 

ness  of  the  wall,  and  h  =  the  height,  the 
conditions  of  simple  stability  will  be  ful- 
filled when 


2  6  ' 

and  solving  for  x,  we  get 

-i^'i ^'\ 

the  thickness  of  the  wall  =  4  ft.  4  in.* 
A  simple  example  has  been  selected  for 

*  It  will  grreatly  facilitate  all  such  calculations  to 
make,  once  for  all,  certain  reductions  and  simplifica- 
tions :  Thus,  the  weisrht  of  a  cubic  foot  of  water 
being  62.5  lbs.,  the  thrust  against  a  reservoir  wall  of 
height  h,  sustaining  a  body  of  water  level  with  its  top 
is,  per  running  foot,  31.25^2,  and  the  moment  of  the 
same  is  10.42^3.  Also  the  thickness  x  of  a  rectangular 
wall  of  density  tt  per  cubic  foot,  in  exact  equili- 
brium with  this  moment,  is  given  by  the  equation, 

^  _  _  _.  This  shows  that  ;r  varies  inversely  as  the 
^/'^  square  root  of  the  density.  It  will  be 
found  by  trial  that  this  formula  gives  the  same  result 
as  (2),  the  only  difference  being,  that  in  the  one  just 
given,  all  the  roots  have  been  extracted,  except  the 
root  of  the  density  of  the  masonry. 

A  good  authority---Mr.  Fanning— gives  as  a  proper 
approximate  thickness  for  a  rectangular  wall  of 
density  =  140  lbs.  per  cubic  foot,  .r  =  0.55A.  It  will 
be  perceived  that  this  is  about  40  »,^  additional  beyond 


56 

illustration  ;  but  of  course  a  rectangular 
section  of  wall  would  not  be  found  gen- 
erally ajDplicable  in  practice,  nor  would  it 
be  convenient  to  limit  the  dimensions  of 
a  retaining  wall  of  whatever  kind  to  the 
minimum  that  would  sustain  the  pres- 
sure. If  this  principle  of  calculation  be 
applied  to  ascertain  the  stability  of  a 
bank  of  earth  with  long  slopes  of  2^  or 
3  to  1,  it   can   easily   be   shown  that   in 


exact  equilibrium.  If  we  should  make  x  =  0.60  h,  we 
would  add  50%,  and  increase  the  moment  of  the  wall 
by  19%,  while  the  section  would  be  increased  only 
9%,  for  the  moments  increase  as  the  squares  of  the 
bases,  and  the  sections,  as  the  bases,  only. 

Such  walls  are  not  usually  built  with  a  rectangular 
section,  but  the  best  way  to  design  a  trapezoidal  wall 
is  first  to  determine  the  thickness  of  a  rectangular 
wall  with  the  desired  coefficient  of  safety,  and  of  the 
given  height,  and  then  transform  it  into  one  of 
trapezoidal  section,  having  an  equal  moment  of 
resistance.  This  may  be  readily  done  by  using  Vau- 
ban's  rule,  which  is,  that  all  walls  with  vertical  backs 
and  of  equal  resistance,  have  the  same  thickness  at 
l-9th  of  their  height,  to  a  very  close  approximation, 
and  within  extended  limits.  Thus,  suppose  we 
wished  tt)  transform  a  rectangular  wall  27  feet  high 
and  15  ft.  thick,  into  an  e(iuivalent  trapezoidal  wall 
with  vertical  back,  and  top  thickness  of  6  feet.  Join 
the  outer  extremity  of  the  top  with  a  point  3  feet  above 
the  base  of  the  outer  face,  and  prolong  the  line  till  it 
strikes  the  base  prolonged.  We  shall  then  have  a  wall 


57 

every  case  the  resistance  of  the  bank  to 
overturning  is  greatly  in  excess  of  the 
horizontal  leverage  exercised  by  the 
water  sustained. 

The  only  theory,  then,  in  any  degree 
tenable,  is  that  assuming  a  bank  in  yield- 
ing to  the  pressure  of  water  to  slide  on 
its  base.  In  order  to  conceive  how  this 
can  apply,  it  is  necessary  to  assume  the 
embankment  to  be  a  rigid  body  resting, 
for  a  given  length  of  its  section,  on  a 
horizontal  plane;    and  without  any   ad- 

27  feet  high,  top  thickness  6  feet,  bottom  thickness 
16.125  feet,  with  a  face  batter  of  4]4  inches  to  the  foot. 
The  moment  of  resistance  of  this  wall  is  slightly  in  ex- 
cess of  that  of  the  assumed  rectangular  wall,  while 
its  section  is  less  by  more  than  26  y^. 

Reservoir  walls  of  considerable  length— say  over 
five  times  their  height— should  be  reinforced  by  ex- 
terior counterforts,  or  buttresses,  not  further  apart 
than  the  height  of  the  wall,  and  carried  up  to  at  least 
half  its  height.  The  section  of  the  wall  should  not  be 
diminished  on  account  of  these  buttresses,  which  are 
merely  intended  to  give  the  long  wall  the  same 
strength  that  a  shorter  one  of  the  same  section  would 
possess.  In  all  these  calculations  the  adhesion  of  the 
mortar  is  neglected.  Perhaps  this  omission  is  counter- 
balanced by  the  assumption  that  the  wall  is  a  mono- 
lith. If  we  wished  to  take  account  of  the  adhesion  of 
the  mortar,  probably  the  best  way  would  be  to  con- 
sider its  intensity  in  pounds  per  square  inch,  as  so 
much  additional  weight  added  to  that  of  the  wall. 


hesion,  or  a  very  small  fraction,  existing 
between  the  surfaces  pressed.  The 
amount  of  the  friction,  however,  is  just 
the  point  upon  which  the  whole  matter 
hinges,  and  until  it  has  been  ascertained 
that  the  surfaces  of  earth  that  are  care- 
fully incorporated  with  one  another  have 
any  such  thing  as  a  coefficient  of  fric- 
tion, it  is  idle  to  pursue  the  investiga- 
tion by  a  mathematical  mode  of  reason- 
ing. The  conditions  of  stability  will  be 
satisfied  when  the  horizontal  component 
of  the  water's  pressure  against  the  bank 
will  equal  the  weight  of  the  bank,  plus 
the  vertical  pressure  exercised  by  the 
water  to  hold  it  down  and  multiplied  by 
the  coefficient  of  friction  ;  but  nothing  is 
known  of  this  coefficient,  and  conse- 
quently the  equation  remains  incapable 
of  solution.  As  a  matter  of  fact,  em- 
bankments do  not  slide  bodily  forward 
on  their  base  when  they  fail,  but  give  way 
from  other  causes  than  mathematical 
reasoning  can  supply.  Landshps,  it  is 
true,  to  some  extent  support  the  principle 
that  maintains  the    sliding    of   embank- 


59 


ments;  but,  here,  the  circumstances  are 
widely  diiferent.  LandsHps  either  take 
place  when  a  mass  of  earth  rests  upon  an 
inclined  surface  of  rock,  with  an  ample 
supply  of  water  to  lubricate  the  surfaces 
in  contact,  or  else  they  are  the  result  of 
cutting  or  embanking  earth  to  a  higher 
slope  than  the  material  will  stand  at ;  the 
infiltration  of  water  also  in  this  case  is 
the  chief  agent  in  producing  the  effect, 
acting  as  a  lubricant,  and  causing  the 
earth  to  assume  its  natural  slope.  In 
each  case  the  surface  of  separation  is  an 
inclined  plane,  an  element  that  does  not 
enter  into  the  question  of  the  stability  of 
embankments,  by  either  of  the  modes  of 
reasoning  above  referred  to.  The  prin- 
ciples that  direct  the  design  of  embank- 
ments to  retain  water  are  not  those  that 
apply  to  the  calculation  of  the  forces  to 
be  resisted  or  the  means  to  overcome 
them,  any  more  than  breakwaters  and 
harbor  walls  can  be  designed  on  math- 
ematical principles.  The  whole  question 
naturally  turns  on  what  slope  the  ma- 
terial composing  the  bank  will  stand  at- 


60 

If  earth  could  be  got  to  remain  at  a  slope 
of  1  to  1,  even  though  the  embankment 
had  no  thickness  whatever  at  top,  it 
would  be  amply  sufficient  in  weight  to 
uphold  the  water  in  a  reservoir.  This, 
however,  cannot  be  accomplished  without 
the  assistance  of  retaining  walls,  which 
would  be  found  in  most  cases  much  more 
expensive  than  the  additional  earth  re- 
quired to  increase  the  slope  to  the  angle 
of  stability ;  and  therefore  the  section  is 
so  disposed  that  the  earth  shall  stand 
both  inside  and  outside  the  reservoir  at 
such  a  slope  as  will  be  under  all  circum- 
stances permanent.  These  slopes  have 
been  determined  by  long  practice  and  by 
success  and  failure  in  pre-existing  in- 
stances— that  is  to  say,  the  limits  have 
been  laid  down,  for  it  is  not  to  be  as- 
sumed that  all  descriptions  of  earth  will 
fall  to  exactly  the  same  slope  when  ex- 
posed to  the  constant  action  of  water  or 
weather.  Earth  when  subjected  to  the 
contact  of  water  almost  invariably  loses 
a  certain  amount  of  its  stability,  and 
therefore  it  is  usual  to  give  the  inner  side 


61 

of  an  embankment  a  longer  slope  than 
the  outside.  In  most  of  the  best  exist- 
ing examples  the  inside  slope  of  the  bank 
is  either  3  to  1  or  2J  to  1,  and  it  is  rare 
to  meet  any  departure  from  this  rule. 
The  outside  slope  may  be  designed  at 
from  2  to  1  to  3  to  1,  depending  upon 
the  character  of  the  material,  its  power 
of  withstanding  the  erosive  action  of  the 
air,  and  the  means  used  to  protect  the 
surface  from  being  washed  off  or  from 
crumbling  away.  In  designing  embank- 
ments, the  impermeability  of  the  earth  is 
a  matter  that  cannot  be  relied  upon. 
There  are,  it  is  true,  innumerable  em- 
bankments now  standing  that  have  never 
allowed  the  escape  of  a  drop  of  water 
from  the  reservoir,  although  no  special 
precaution  was  taken  to  make  them  water- 
tight. Of  these  India  abounds  with  ex- 
amples, the  introduction  of  a  puddle  wall 
being  in  the  older  embankments  of  very 
exceptional  occurrence.  The  earth  was 
merely  dug  out  close  at  hand,  and  carried 
by  the  work-people  in  baskets  on  their 
heads  to  where  it  was  deposited,  without 


6r> 


any  regard  to  the  mode  of  disposing  the 
material.  The  author  has  had  occasion 
to  construct  a  considerable  length  of 
levee,  or  embankment,  on  this  simple 
plan  for  the  protection  of  the  country 
from  the  flooding  of  a  river;  and 
although,  so  far  as  he  is  aware,  no  flood 
has  yet  taken  place  to  test  the  work, 
he  has,  from  the  study  of  existing  ex- 
amples, entire  confidence  in  the  result. 
The  earth,  so  far  as  practicable,  was  dis- 
posed in  layers,  and  before  each  was  com- 
pleted it  was  thoroughly  consolidated 
by  the  tread  of  the  workmen.  It  is  not 
suggested  that  the  puddle  wall  should  be 
dispensed  with  in  designing  embank- 
ments, for  the  additional  degree  of  safety, 
in  most  instances,  will  more  than  compen- 
sate for  the  extra  expense  it  entails  ;  but, 
in  low  embankments  made  of  good  re- 
tentive clay,  the  precaution  of  puddling 
is  by  no  means  a  necessity. 

In  most  of  the  best  examples  of  em- 
bankments in  England,  the  practice 
adopted  has  been  to  carry  up  the  earth- 
work in   layers  of  2  or  3  ft.   in  thickness, 


63 


64 


65 

disposed  in  the  manner  shown  in  Figs. 
6  and  7,  and  at  the  same  time  to  con- 
struct in  the  center  of  the  bank  a  wall  of 
well-puddled  clay,  the  foundation  of  which 
is  carried  down  for  whatever  depth  may 
be  necessary  in  order  to  reach  an  imper- 
meable bed  of  earth  or  rock.  It  is  not  in 
all  situations  possible  to  procure  earth 
exactly  suitable  and  in  sufficient  quantity 
for  the  construction  of  an  embankment, 
and  consequently  it  is  usual  and  advis- 
able to  dispose  the  best  part  of  the  ma- 
terial— that  is,  the  most  retentive  of 
water — in  juxtaposition  to  the  puddle 
wall,  as  indicated  in  Fig.  6.*  In  this  ex- 
ample, the  selected  material  is  disposed 
equally  at  either  side  of  the  puddle ;  but, 
as  its  function  is  to  withstand  the  ad- 
mission of  water,  it  would  probably  be 
more  consistent,  though  less  in  accord- 

*  This  practice  is  sound,  but  would  iu  many  cases 
be  very  diflflcult  of  execution,  at  least  without  con- 
siderable extra  expense.  When  the  borrow-pits  are 
opened,  the  material  is  generally  taken  as  it  comes. 
The  top  soil,  and  all  roots  and  sods  should  be  first 
removed,  and  all  stones  larger  than  are  allowed  by 
the  specifications  are  picked  out  and,  generally,  re- 
erved  for  the  rip-rapping  of  the  bank. 


66 


67 


ance  with  practice,  to  place  all  the  select- 
ed   material   on   the   inner    side.      The 
practice  of  excavating  the  earth  for  an 
embankment  from  the  inside  of  the  reser- 
voir is  one  that  should  not    be  followed 
without  caution.      Removing  so  large  a 
mass  of  material  would,  no  doubt,  give  a 
considerable  increase  of  storage  room  5 
but  sometimes  the  bed  of  a  reservoir  is 
covered  by  a  layer    of  impervious    clay 
that  is  of  immense  value,  and  if  this  be 
cut    through    or    removed,    it   is   quite 
possible  that  a  bed  of  porous  material 
may  be  met  with  sufficient  to  allow  the 
escape  of  water  when  it  comes  to  be  ad- 
mitted.    In  specifying  for  the  dimensions 
of  the   puddle   wall,  a    sound   rule   for 
adoption  is,  that  it  shall  have  a  thickness 
of  10  ft.  at  the  top  water-line  and  in- 
crease in  thickness  to  the  surface  of  the 
ground  at  the  rate  of  1  in.  on  each  side 
for  every  foot  of  height.     Before  any  ex- 
cavation is  commenced,  it  will  be  essen- 
tial  to   make    a    sufficient    number    of 
borings  to  ascertain  the  nature  of  the  soil 
beneath  the  surface. 


68 

It  may  here  be  mentioned  that  profes- 
sional men  are  not  apparently  agreed  as 
to  the  principles  to  be  kept  in  view  in 
constructing  reservoir  embankments ;  and 
this  want  of  concurrence  never  was  more 
apparent  than  in  the  discussion  that  fol- 
lowed the  destruction  of  the  Dale  Dyke 
reservoir,  near  Sheffield.  Fig.  8  shows 
a  plan  of  the  embankment  site  after  the 
catastrophe.  The  bank  was  95  ft.  high, 
with  slopes  of  2^  to  1,  and  a  top  width  of 
12  ft.  The  puddle  wall  was  16  ft.  in 
width  at  the  ground-line,  and  tapered  to 
4  ft.  at  the  top  of  the  bank.  This  em- 
bankment, with  the  exception  of  the 
puddle  wall,  was  composed  of  rubble 
stone  and  shale  ;  an  additional  price  hav- 
ing been  given  by  the  engineers  to  insure 
the  use  of  the  former  material ;  which 
proves,  at  any  rate,  that  this  mode  of 
construction  was  adopted  on  principle 
and  not  through  ignorance  or  mistake. 
From  the  evidence  given  by  the  engineers 
of  the  company,  it  appears  that  it  was, 
in  their  opinion,  desirable  that  the  inner 
part  of  the  embankment  should  be   per- 


69 


SheFa^ 


70 

meable  to  water,  because  earth  was  much 
more  Hkely  to  subside  and  slip  than  an 
open  and  less  yielding  material  like  stone. 
This  mode  of  construction  implies  that 
the  puddle  shall  be  fully  sufficient  of  itself 
to  resist  the  passage  of  water,  and  that 
there  is  no  necessity  to  relieve  it  of  any 
part  of  the  pressure  against  it.  Of  course, 
if  a  bank  be  composed  of  open  work, 
every  point  in  the  face  of  the  puddle  is 
exposed  to  the  full  and  direct  hydrostatic 
pressure ;  and  if  at  any  point  there  is  the 
smallest  fissure  or  imperfection,  the  water 
has  full  power  against  it,  and  will,  to  a 
certainty,  take  advantage  of  such  point  to 
breach  the  dam.  The  assumption,  then, 
of  the  constructors  of  this  and  the  Agden 
reservoir  evidently  was  that  a  puddle  wall 
of  some  25,000  sq.  ft.  of  area  was  to  be 
constructed  without  an  imperfection  of 
any  kind,  or  a  single  weak  point  in  the 
whole  surface. 

The  obvious  reason  for  employing 
puddle  at  all,  in  embaukments,  is  to 
thoroughly  close  up  any  imperfection 
that  may  occur  in  the  earthwork ;  it  is  in 


71 


fact  merely  an  accessory,  and  cannot  be 
relied  upon  of  itself  to  secure  the  em- 
bankment against  destruction.  If  an 
embankment  be  constructed  of  good 
sound  earthwork,  j^roperly  executed,  it 
is  highly  probable  that  the  water  may 
never  penetrate  half  way  through  to  the 
puddle  wall,  and  probably,  in  the  majority 
of  examples,  has  not  done  so.  Earth- 
work, however,  is  not  always  executed 
without  imperfection;  some  decompos- 
able material  maybe  introduced,  which,  in 
course  of  time,  dissolves,  leaving  a  fissure ; 
one  part  may  be  at  first  less  consolidated 
than  another,  and,  subsiding,  lead  to  im- 
perfection ;  or  an  embankmeut,  be  it  ever 
so  well  constructed,  may  be  burrowed 
through  by  moles,  rats,  and  other  vermin. 
It  is  to  meet  the  first  two  of  these 
sources  of  imperfection  that  puddle  is 
used;  and  if,  by  such  fissures  as  may 
occur  in  ordinary  eartli-work,  water  is  ad- 
mitted as  far  as  the  puddle  wall,  it  can 
only  exercise  pressure  against  it  at  a  few 
points,  the  puddle  and  earth  being,  iu 
good  work,  so  bonded  and  incorporated 


72 


with  each  other  that  there  is  no  space 
left  for  the  water  to  occupy  and  press 
against  the  surface.  Most  who  have  read 
the  account  of  the  disaster  that  oc(?urred 
in  March,  1864,  at  Sheffield,  will  recollect 
how  singularly  conflicting  the  profes- 
sional evidence  on  that  occasion  was. 
Some  of  the  first  engineers  were  ranged 
against  each  other  in  order  to  satisfy  the 
public  as  to  whether  the  failure  of  the 
embankment  was  attributable  to  bad  en- 
gineering or  to  a  landslip  ;  and  although 
the  impression  finally  remained  on  the 
public  mind  that  *'  there  was  not  that  en- 
gineering skill  and  attention  to  the  con- 
struction of  the  works  that  their  magni- 
tude and  importance  demanded,"  the  en- 
gineers were  fairly  divided  in  opinion  as 
to  the  cause  of  the  disaster.  One  section 
pronounced,  without  quahfication,  that 
the  embankment  gave  way  in  consequence 
of  a  landslip,  and  entirely  ignored  the 
fact  of  the  embankment  being  defectively 
constructed ;  whilst  the  other  gentlemen 
gave  their  verdict  dead  against  the  com- 
pany, and   their    mode  of   constructing 


73 

water-tight  banks.*  The  two  diagrams, 
Nos.  6  and  7,  may  be  taken  as  indicating 
the  system  of  constructing  embankments 
most  generally  approved  of.  The  puddle^ 
as  will  be  observed,  is  carried  up  to  the 
natural  surface  of  the  ground  without 
any  batter,  and  from  that  point  slopes  on 
each  side  to  the  top  of  the  bank;  on 
either  side  of  the  puddle  is  disposed,  in 
concave  layers,  the  most  sound  and  re- 
tentive part  of  the  material,  and  outside 
of  all  comes  the  ordinary  earthwork.! 

As  a  security  against  the  eroding  action 
of  the  water,  and  also  against  the  inroads 
of  vermin,  the  most  desirable,  as  well  as 
the  most  usual  practice,  is  to  pitch  the 
whole  of  the  inner  surface  of  an  embank- 
ment with  stone,  carefully  laid  by  hand. 
Neglect  of  this  precaution  has  led  to  the 
destruction  of  many  embankments  in 
other  respects  securely  constructed,  and 
even  when  ample  height  of  bank  above 

*  At  the  present  day,  it  seems  incredible  that  any 
other  opinion  should  have  been  entertained. 

t  It  may  be  doubted  if  disposing  of  the  material  in 
concave  layers  is  good  practice. 


74 


the  surface  of  highest  water  was  provided. 
In  all  ordinarily  inclement  weather  the 
disturbance  of  the  surface  of  a  reservoir 
amounts  to  no  more  than  a  mere  ripple  ; 
but  when  the  surface  is  of  large  extent, 
and  a  severe  storm  blowing,  the  waves 
produced  are  such  as  to  cause  reasonable 
apprehension,  and,  in  fact,  have,  before 
now,  overtopped  the  bank  and  cut  it 
down,  till  the  water  flowed  over  and 
caused  the  destruction  of  the  work.  In 
most  cases,  it  will  be  necessary  to  leave 
about  5  ft.  between  the  level  of  the  high- 
est water  and  the  top  of  the  embankment, 
and  never  less  than  3  ft. 

A  mode  of  construction  not  very  gen- 
erally used,  but  apparently  consistent 
with  reason,  is  that  shown  in  Fig.  7,  the 
embankment  for  the  Biddeford  Water- 
works. It  consists  in  covering  the  whole 
of  the  inner  face  with  a  layer  of  puddle, 
with  sometimes  a  layer  of  peat  outside  it. 
On  some  occasions  it  has  been  thought 
de&irable  to  mix  with  the  puddle  a  quan- 
tity of  small  stones  or  furnace  cinders, 
by  way  of  obstruction  to  vermin — a  pre- 


75 


caution  that  is  bj  no  means  unnecessary. 
As  an  instance  in  point  the  author  is  re- 
minded of  a  masonry  dam  in  India  that 
had  to  be  pointed  every  year  regularly, 
because  the  fresh-water  crabs  in  the 
reservoir  found  it  convenient  and  pro- 
motive of  their  development  of  shell  to 
appropriate  the  mortar  to  their  personal 
use.  The  joints  were  cleaned  out  as  effec- 
tually at  the  end  of  each  monsoon  as  if  the 
work  had  been  done  to  order. 

The  preparation  of  the  foundation  for 
an  embankment  is  a  matter  requiring 
some  care.  The  soil,  consisting  of  grass, 
roots,  etc.,  and  other  matters  of  a  decom- 
posable nature,  should  be  carefully  re- 
moved over  the  whole  surface  to  be  cov- 
ered by  the  bank,  and  if  any  porous  ma- 
terial, such  as  sand  or  gravel,  be  present, 
it  must  be  removed,  until  a  compact  and 
water-tight  bed  is  arrived  at.*     The  bank 


*  This  must  be  taken  with  some  reserve.  It  is  evi- 
dent that  carryinfj  out  this  n  commendation  hteraiiy, 
would  aiuouut  to  dijigin^  out  tiie  whole  site  of  the 
embankment  to  a  depth  tqual  to  tliat  of  the  center- 
wall  louiidation.  All  that  can  be  done  in  most  cases, 
is  to  remove  the  sods,  roots,  tic,  from  the  site  of  the 
eiuOankment,  so  ttiat  a  fresh  earth  surface  is  laid 
bare,  and  commence  the  till  upon  that. 


76 

must,  in  fact,  be  in  contact  with  some 
sound  and  reliable  material  that  will  not 
admit  the  passage  of  water. 

APPENDAGES  OF  RESERVOIRS. 

Under  this  heading  may  be  considered  : 

The  whole  apparatus  for  allowing  the 
water  to  escape,  including  the  pipes,  the 
valve  tower,  and  the  culvert. 

The  waste  sluices. 

The  waste  weir  or  by- wash. 

The  most  economical  mode  of  discharg- 
ing water  from  a  reservoir  is  through  a 
single  pipe  passing  either  through  the 
embankment  or  immediately  under  it ; 
but  this  plan  cannot  under  any  circum- 
stances, be  recommended,  though  it  is 
some  times  found  in  existing  examples. 
It  is  open  to  several  grave  objections,  the 
principal  of  which,  perhaps,  is  that  the 
failure  of  a  joint  under  the  embankment 
from  unequal  pressure,  or  from  whatever 
cause,  will  probably  produce  the  destruc- 
tion of  the  embankment,  or  at  any  rate, 
entail  a  serious  interruption  to  the  sup- 
ply, by  the  reservoir  having  to  be  emptied, 
in  order  to  repair  the  pipe.     Buried  in  or 


77 


under  an  embankment,  a  pipe  is  com- 
pletely out  of  reach  and  out  of  view,  and 
may  be  in  a  very  defective  state  with- 
out its  being  possible  to  detect  the  im- 
perfection. 

In  order  to  secure  the  satisfactory 
working  of  a  reservoir  as  a  source  of  con- 
stant supjDly,  it  is  essential  that  the  out- 
let pipes,  valves,  and  all  other  append- 
ages for  controlling  and  regulating  the 
escape  of  the  water,  should  be  accessible 
for  inspection  and  repair.  The  usual 
mode  of  accomplishing  this  is  to  carry 
the  pipes  out  through  a  culvert  of  brick 
or  masonry  of  sufficient  dimensions  to 
admit  a  man.  This  culvert  communi- 
cates with  the  valve  tower,  as  shown  in 
Figs.  6  and  7,  so  that  there  is  a  complete 
communication  between  the  outside  of 
the  reservoir  and  the  inside.  When  un- 
avoidable, the  culvert  is  carried  straight 
under  the  embankment  in  the  natural 
ground ;  but  the  safest  and  most  gener- 
ally approved  mode  of  construction  is  to 
bring  the  culvert  round  the  end  of  the 
embankment,  where  it  will  be  out  of  reach 


78 


of  injury  from  unequal  settlement ;  a 
source  of  no  small  apprehension  when 
either  culvert  or  pipes  alone  are  carried 
under  the  bank.  Where  possible,  it  is 
an  excellent  plan  to  run  a  heading 
through  the  solid  ground,  lining  it  with 
brickwork  and  puddling  it,  forming  a 
tunnel  entirely  independent  of  the  em- 
bankment. The  principal  objection  to 
carrying  either  the  culvert  or  pipes 
through  or  under  the  bank  is  their 
liability  to  fracture  from  the  enequal 
settlement  of  the  earthwork.  It  would 
appear  that  their  liability  to  damage  can- 
not with  certainty  be  insured  by  any 
reasonable  depth  of  excavation,  and  is, 
therefore,  generally  disapproved  of  by 
the  best  authorities. 

In  the  best  constructions  the  culvert  is 
situated  half  way,  or  two-thirds  up  the 
embankment,  and  in  such  case  the  outlet 
pipes  for  drawing  off  the  water  in  the 
reservoir  act  as  syphons  when  the  water 
surface  has  fallen  below  the  culvert. 
Fig.  6  shows  a  plan,  as  well  as  a  cross 
section,  of  a  reservoir  dam  designed  for 


79 


general  application  by  Mr.  Rawlinson. 
Here  the  bottom  of  the  culvert  is  about 
25  ft.  above  where  the  inner  slope  of  the 
embankment  intersects  the  ground  at 
the  lowest  point.  The  syphon  pipe  is 
also  shown  passing  through  the  culvert ; 
the  horizontal  culvert  is  connected  with 
a  shaft  inside  the  embankment,  in  which 
are  placed  the  valves  for  leading  off  the 
supply  from  the  reservoir.  The  valves 
are  made  to  be  closed  on  the  inside  by 
valve  spindles  and  screws,  and  the  inlet 
pipes  are  closed  on  the  outside  by  plugs 
which  can  be  applied  from  the  top  of  the 
valve  tower.  Thus  the  engineer  has  full 
command  of  the  whole  of  the  outlet 
works ;  all  the  pipes  and  valves  are  easily 
accessible  and  under  perfect  control,  so 
that  the  supply  can  at  any  time  be 
arrested  for  the  repair  of  any  derange- 
ment that  may  occur,  even  to  the  removal 
and  replacement  of  all  the  pipes.  The 
inlet  pipes  are  shown  in  this  example,  as 
well  as  in  Fig.  7,  fixed  at  different  heights 
in  the  valve-tower,  the  object  of  which  is 
to  draw  the  supply    from    the    reservoir 


80 


from  points  near  the  surface.  The  out- 
let pipe,  passing  through  or  under  the 
embankment,  may  be  connected  on  the 
inside  of  the  reservoir  by  a  flexible 
joint  with  another  pipe  of  the  same 
diameter,  to  the  upper  end  of  which  is  at- 
tached a  float.  This  pipe  is  movable  in  a 
vertical  plane,  being  controlled  from 
lateral  motion  by  the  guide-posts.  Such 
an  arrangement  admits  of  the  water  be- 
ing drawn  off  from  the  surface,  -where  it. 
is  least  liable  to  be  contaminated  with 
impurities.  Whatever  arrangement  be 
selected  for  drawing  the  supply  off  from 
a  reservoir,  the  system  of  carrying  the 
pipes,  either  with  or  without  a  culvert, 
through  or  under  the  embankment,  can- 
not be  sufficiently  deprecated  ;  they  are, 
in  such  a  position,  beyond  the  reach  of 
inspection,  and,  moreover,  are  very  likely 
to  induce  leakage  from  the  reservoir.  It 
is  usual  to  puddle  carefully  the  culvert 
or  pipes  when  carried  under  or  through 
the  bank,  but,  even  with  such  a  pre- 
caution, the  water  has  under  a  consider- 
able head  a  tendency  to  creep  along  the 


81 

pipe,  and,  by  soaking  into  the  earthwork, 
may  cause  any  one  of  the  many  evils  that 
imperil  and  destroy  embankments. 

When  embankments  are  not  of  great 
height,  an  exceedingly  cheap  and  simple 
mode  might  be  adopted  for  drawing  off 
the  water.  This  would  be  by  laying  a 
syphon  over  the  embankment,  as  was 
done  in  the  case  of  the  middle-level  drain- 
age in  Cambridgeshire,  which  syphon 
would  at  the  inner  side  have  a  flexible  con- 
nection with  another  tube  having  a  float 
attached,  as  above  described.  Such  an 
arrangement  would  apply  in  princii^le  to 
heights  not  exceeding  30  ft.,  as  the 
pressure  of  the  atmosphere  would  main- 
tain no  greater  height.  In  practice,  how- 
ever, the  syphons  cannot  be  worked  with 
success  at  much  above  20  ft.,  for  it  is 
found  that  after  a  short  time,  the  flow 
becomes  arrested  by  the  collection  of  air 
in  the  upper  part  of  the  syphon,  and  it 
becomes  necessary  to  pump  the  air  out 
constantly,  to  prevent  it  from  interfering 
with  the  flow,  as  it  would  do  if  not  re- 
moved.    It  would  appear  a  simple  mat- 


82 


ter,  where  it  is  desirable  to  adopt  a 
syphon,  to  utihze  the  power  of  the  water 
flowing  out  for  the  purpose  of  getting 
rid  of  the  air ;  it  miglit  easily  be  applied, 
through  a  small  wheel  and  suitable  gear- 
ing, to  work  an  air-pump  fixed  at  the 
highest  point  of  the  syphon,  making  the 
whole  arrangement  self-acting.  The 
arrangement  could  be  successfully  ap- 
plied to  irrigation  tanks  in  India,  where 
the  embankments  are  frequently  less  than 
30  ft.  Each  leg  of  the  syphon  should  be 
provided  with  a  valve  to  retain  the  water, 
and  when  the  supply  was  intermittent  it 
would  be  essential  to  have  an  opening  at 
the  highest  point  of  the  syphon,  and 
some  appliance,  perhaps  an  air-pump,  for 
filling  it  with  water  in  case  of  leakage. 

To  insure  a  constant  discharge  from 
a  reservoir  with  a  constantly  varying 
head,  several  methods  have  been  adopted ; 
of  these,  one  of  the  most  ingenious  is 
that  used  at  the  Gorbals  Waterworks, 
near  Glasgow.  Fig.  9  represents  a 
transverse  section  through  the  regulator- 
house,  showing  the  arrangement  by  which 


83 

the  discharge  is  equalized.  To  the  orifice 
of  the  outlet  pipe,  O,  is  fitted  a  square- 
hinged  flip  v.ilve  of  wood,  against  which 
presses,  by  a  friction  roller,  a  lever,  B, 
the  arms  of  which  are  bent.  To  the 
upper  arm  is  attached  a  chain  that  passes 
over  a  pulley,  and  is  connected  with  a 
cast-iron  cylinder  or  float,  D,  that  stands 
in  the  reservoir,  E,  of  slightly  larger 
diameter.  At  the  side  of  the  entrance- 
door  of  the  building  is  placed  another 
cistern,  G,  of  cast-iron,  closed  at  top,  and 
communicating  by  a  pipe,  R  R,  with  the 
vertical  pipe,  H,  which  is  in  connection 
with  the  outlet  pipe,  and  passes  up  the 
slope  of  the  embankment,  to  carry 
away  any  air  that  may  accumulate 
in  the  main.  The  cistern,  G,  is  con- 
nected with  the  reservoir,  E,  by  a  pipe,  K, 
which  supplies  water  to  float  the  cyl- 
inder, D.  Now,  it  is  evident  that  the 
discharge  from  the  reservoir  will  be  reg- 
ulated by  the  position  of  the  lever,  B, 
and  this  again  will  be  controlled  by  the 
height  of  the  float,  D.  To  regulate  this 
height   the   supply  from  the  cistern,  G, 


84 


85 


must  be  self-adjusting,  or  be  regulated 
by  the  amount  of  water  flowing  away. 
The  float,  N,  has  attached  to  it  a  spindle, 
on  which  are  fixed  two  double-beat 
valves  that  work  in  the  vertical  part  of 
the  pipe,K,  one  of  which  admits  water  from 
the  cistern,  G,  into  the  cylinder,  E,  and 
the  other  allows  the  water  to  escape 
from  the  reservoir,  E.  Now,  if  the  sur- 
face of  the  water  upon  which  the  float, 
N,  rests  should  rise  above  the  proper 
level,  the  float  forces  up  the  spindle, 
closing  the  supply  valve  from  the  cistern, 
and  at  the  same  time  opening  the  lower 
valve.  Thus  the  supply  is  cut  off  and 
the  escape  opened,  enabling  the  float,  D, 
to  fall.  The  subsidence  of  the  float 
closes  more  or  less  the  flap  valve,  and 
checks  the  discharge,  in  consequence  of 
which  the  surface  of  the  water  falls,  and 
with  it  the  float,  N,  which  consequently 
opens  the  supply  valve,  and  again  admits 
water  into  the  cistern,  E.  Thus  an 
almost  perfect  equality  between  the  con- 
sumption and  the  supply  of  water  is  pre- 
served.    It  would  appear  that  the  same 


86 

effect  could  be  produced  by  connecting 
the  lever  directly  with  a  float  on  the  sur- 
face of  the  water,  but  such  an  arrange- 
ment would  only  apply  when  the  pres- 
sure against  the  flap  is  trifling.* 

It  is  essential  that  every  reservoir 
should  be  provided  with  some  means  of 
getting  rid  of  the  excess  of  water  that 
flows  into  it,  and  whether  this  provision 
be  made  by  a  waste  weir,  sluices,  or 
waste  pit,  it  is  one  that  should  not  be 
omitted.  The  most  advantageous  posi- 
tion for  a  waste  weir  will  generally  be  at 
some  point  remote  from,  and  entirely  un- 
connected with,  the  embankment,  and 
occasionally  a  natural  depression  in  the 
ground,  as  shown  in  Fig.  4,  will  afford 
remarkable  facilities  for  the  construction 


♦  As  a  general  rule,  the  discharge  is  regulated  by 
openiDg  the  gates  to  a  greater  or  less  extent,  accord- 
ing as  more  or  less  water  is  required.  Indeed,  it 
would  seem  unwise  to  have  an  automatic  system, 
maintaining  a  constant  discharge  irrespective  of  a 
diminishing  head.  As  the  supply  of  water  in  the 
reservoir  diminishes,  it  is  better  to  let  the  draft  from 
it  diminish  also,  as  a  warning  that  the  reserve  is  being 
drawn  upon. 


87 

of  an  escape.  The  level  of  the  crest  of 
the  waste  weir  with  reference  to  the  top 
of  the  dam  will  require  to  be  carefully 
adjusted,  the  minimum  difference  of  level 
being  3  feet,  and  the  maximum  about 
10  ft.,  depending  on  varying  circum- 
stances. The  height  of  the  waste  weir 
will,  of  course,  regulate  the  top  water 
level  in  the  reservoir ;  and  this  must  be 
fixed  with  regard  to  the  probability  of 
the  embankment  being  over-topped  by 
waves.  The  circumstances  influencing 
the  height  of  the  waves  in  a  reservoir  are 
the  extent  of  the  water  surface,  the 
depth,  and  the  amount  of  exposure  to  or 
shelter  from  wind,  all  of  which  will  vary 
with  each  particular  case.  Under  or- 
dinary circumstances,  the  height  of  the 
top  of  the  embankment  above  the  crest 
of  the  waste  weir  should  be  for 

an  embankment  25  ft.  deep,  4  ft. 
50  ft.  "  5  ft. 
75  ft.      ''      6  ft. 

and  for  greater  height  of  embankment 
the  difference  of  level  may    be  propor- 


88 


tionately  increased.  When  the  configura- 
tion of  the  ground  does  not  afford  any 
facilities  for  the  construction  of  a  waste 
weir  after  the  manner  described,  suffi- 
cient provision  for  the  escape  of  the 
overflow  is  made  through  a  waste  pit. 
This  waste  pit,  or  tower,  is  generally  a 
circular  structure  built  over  the  outlet 
culvert  inside  the  reservoir,  and  serves 
equally  for  access  to  the  valves  and  for 
the  escape  of  the  flood  water.  With 
regard  to  the  capacity  of  the  waste  weir 
or  waste  pit,  whichever  be  adopted,  it 
will  be  necessary  to  make  ample  provision 
for  the  discharge  of  the  sudden  acces- 
sions of  flood  water  that  reservoirs  are 
subject  to,  and  which  so  seriously  im- 
peril their  safety.  To  provide  for  this 
there  is  an  empirical  rule  amongst  en- 
gineers that  is  supposed  to  suffice  for 
the  most  urgent  contingencies.  It 
states  that  there  shall  not  be  less  than  3 
ft.  of  length  of  overfall  for  every  hundred 
acres  of  gathering  ground,  but  it  is 
obvious  that  to  proportion  the  length  of 


89 


the  waste  weir  to  a  given  area  of  country 
in  all  cases  would  be  unreasonable.* 

The  discharge  over  the  weir  will  not 
depend  only  upon  the  quantity  of  rain 
falling  on  a  certain  area  of  ground,  but 
also  on  the  extent  of  the  reservoir  as 
compared  to  the  gathering  ground,  and 
on  the  flat  or  precipitous  character  of  the 
basin.  The  only  safe  mode,  then,  of 
proportioning  the  length  of  the  escape 
will  be  to  ascertain  with  exactness  what 
the  discharge  of  the  stream  or  streams 
flowing  out  of  the  reservoir  was  during 
the  greatest  known  flood,  and  then  fixing 
upon  an  arbitrary  depth  for  the  water  to 
flow  over  the  weir,  say  2  ft.  or  3  ft.,  to 
calculate  what  length  of  overfall  will 
suffice  for  the  discharge  of  the  excess 
water.  In  India,  where  large  waste- 
weir  accommodation  is  essentially  neces- 

*  The  above  rule  might  answer  up  to  say  3  square 
miles  of  drainage  area,  or  gathering  ground.  Beyond 
this  it  would  give  excessive  lengths.  For  large  areas, 
up  to  say  50  square  miles,  3  ft.  per  square  mile 
would  be  nearer  the  mark.  A  rule  that  would  fit-, 
fairly  well,  all  cases,  would  be :  Length  in  feet  =  20 
times  the  square  root  of  the  number  of  square  miles 
of  drainage  area. 


90 

sary,  while  it  is  equally  a  necessity  to 
save  every  gallon"  of  water  that  is  pos- 
sible, it  is  a  common  practice  to  form  a 
temporary  dam,  of  earth  and  sods,  on 
the  top  of  the  waste  weir ;  this  serves  to 
pond  up  some  3  ft.  or  4  ft.  of  water  over 
the  whole  surface  of  the  reservoir,  and 
does  not  imperil  the  security  of  the 
works.  In  times  of  heavy  floods  the 
water  rises  and  overtops  the  temporary 
dam,  and  no  sooner  does  so,  than  the 
whole  is  carried  away,  and  the  water  in 
the  reservoir  quickly  subsides. 

In  works  designed  for  the  supply  of 
towns,  it  is  sometimes  necessary  to  make 
provision  to  arrest  the  entrance  of  flood- 
water  into  the  reservoir,  as  the  streams 
may  come  down  charged  with  large 
quantities  of  matter  in  suspension  that 
would  injure  the  purity  of  the  water  for 
domestic  consumption.  These  streams 
may  be  diverted  and  carried  round  the 
margin  of  the  tank  past  the  dam,  and 
can  be  admitted  into  the  channel  of  the 
stream,  or  be  utilized  for  mill  power. 
On  tbe  Manchester  Waterwoiks,  are  con- 


91 


structed  across  the  mountain  streams, 
weirs  of  an  ingenious  design,  for  the  pur- 
pose of  separating  the  flood-waters  from 
the  ordinary  flow.  The  dimensions  are 
adjusted  from  observations  of  each  par- 
ticular stream,  so  that  the  discharge  up 
to  a  certain  amount  will  take  place  into 
the  channel  for  the  supply  of  the  town ; 
but  when  the  discharge  increases,  and 
the  water  becomes  turbid,  it  has  suffi- 
cient velocity  to  carry  it  over  the  open- 
ing, and  flows  down  to  the  compensation 
reservoir  for  the  supply  of  mill  power. 

In  determining  the  dimensions  of  a 
weir  of  this  kind  it  is  first  to  be  ascer- 
tained what  the  mean  velocity  of  the 
water  flowing  over  will  be  for  a  given 
depth  of  water,  A,  above  the  crest.  The 
mean  velocity,  v,  will  be 

V  =  \x  8.024^^"  =  5M^h. 
o 

If  the  vertical  height  of  the  crest  of  the 
weir  above  the  point  to  be  overleaped  by 
the  cascade  be  called  x,  the  distance 
across  will  be 


9^ 


2  v^Jx     4    , — 

Before  concluding,  it  will  be  well  to 
give  a  brief  consideration  to  the  causes 
tending  to  the  failure  of  embankments. 
The  foregoing  remarks  will,  in  suggest- 
ing the  best  mode  of  construction,  have 
anticipated  much  that  might  be  said  on 
the  subject  of  failures ;  but  there  are  a 
few  points,  the  recapitulation  of  which 
the  importance  of  the  subject  demands. 

There  are  unfortunately  on  record, 
accidents,  if  they  can  be  so  called,  from 
the  bursting  of  embankments,  that,  if 
estimated  by  the  loss  of  life  attending 
them,  are  as  appalling  as  anything  within 
the  memory  of  man.  Thousands  of 
human  lives  have  been  sacrificed  to  ig- 
norance and  false  economy,  as  well  as  in 
some  instances  to  natural  defects  that  it 
would  have  been  difficult  to  foresee. 

The  existence  of  springs  on  the  site  of 
an  embankment  is  an  undoubted  cause 
for  apprehension,  and  considerable  care 
should  be  taken  to  carry  all  water  from 


93 

this  source  away;*  that  it  may  not,  as 
it  certainly  will  if  not  checked,  force  its 
way  between  the  surface  of  the  ground 
and  the  seat  of  the  embankment.  In 
doing  so  there  is  every  probability  that 
the  earth  of  the  embankment  will  be 
washed  out  by  constant  trickling,  till  a 
fissure  is  formed  of  sufficient  dimensions 
to  render  the  destruction  of  the  bank  a 
certainty,  if  the  water  from  the  reservoir 
should  ever  penetrate  so  far.  As  a  pro- 
vision against  this  source  of  injury,  all 
springs  found  on  the  site  of  an  embank- 
ment should  be  taken  up  and  carried 
away  in  proper  drains  sufficiently  and 
securely  puddled.  Thus  the  water  is 
confined  to  a  single  channel,  and  has  no 
tendency  to  soak  into  the  earthwork  and 
blow  it  up  in  endeavoring  to  escape.  In 
embankments  of  all  kinds  the  presence 
of   water   is   a   most   serious    evil,    and 

*  Where  to?  All  these  springs,  and  all  percolations 
whatever,  must  be  cut  off  and  prevented  from  travers- 
ing the  embankment  by  a  water-iig:ht  puddle  or 
center  wall,  which  is  the  only  safeguard  against  this 
dauirer,  to  meet  which  is  one  of  the  principal  objects 
of  such  wail.  .."I'l.  i  la-jaiii 


94 


one  by  which  may  be  accounted  for,  some 
of  the  most  extensive  land  slips  that  are 
on  record.  It  is  erroneous  to  assume 
that  when  water  is  the  active  element  in 
producing  disruption  in  an  embankment 
or  mass  of  earth  of  any  kind,  that  it  only 
acts  as  a  lubricant  between  the  surfaces 
in  contact.  The  truth  is,  the  bulk  of 
earth  is  sensibly  affected  by  the  amount 
of  moisture  in  it,  as  is  seen  in  the  sub- 
sidence of  newly-formed  railway  banks 
when  exposed  to  rain.  If,  then,  a  suffi- 
cient quantity  of  water  find  its  way  into 
the  center  of  a  bank  that  has  been  put 
together  in  a  comparatively  dry  state,  it 
will  rise  and  soak  into  the  earth  until  at 
length  what  was  a  solid  mass  becomes 
semi-fluid,  settles  into  a  smaller  space 
than  it  before  occupied,  and,  as  a  conse- 
quence, will  leave  a  vacuity  above  it.  The 
inevitable  result  is  the  subsidence  of  the 
superincumbent  earth ;  but  instead  of 
resting,  as  at  first,  on  a  resisting  ma- 
terial, it  floats,  so  to  speak,  on  the  semi- 
fluid mass  underneath,  and  having  little 
or  no  friction  to  overcome,  slips  away  to 


95 

a  lower  angle  than  it  before  stood  at. 
Natural  springs,  therefore,  whenever 
they  occur,  must  be  dealt  with  carefully 
and  completely.  Exactly  similar  effects 
to  those  produced  by  natural  springs 
may  result  from  the  defective  practice  of 
carrying  outlet  pipes  through  or  imme- 
diately under  embankments.  Be  the 
pipes  ever  so  well  puddled,  there  will  be 
a  tendency  to  trickling  along  the  line  of 
their  direction,  and  assuredly  if  this 
trickle  makes  its  way  to  the  center  of  the 
bank  it  will  carry  mischief  with  it.  It  is 
true  that  springs  are  occasionally  found 
issuing  from  the  foot  of  an  embankment, 
without  after  several  years  causing  any 
appearances  to  justify  apprehension.  The 
Doe-park  reservoir  is  an  example  in 
point,  and  though  at  one  time  fears  for 
its  safety  were  entertained,  the  embank- 
ment is  still  standing,  and,  so  far  as  the 
author  is  aware,  the  spr.ng  is  still  trick- 
ling away.  An  engineer  of  eminence  was 
called  upon  to  report  upon  the  state  of 
the  works,  and  gave  his  opinion  that,  as 
the  spring  came  away  without  any  earth 


96 


in  suspension,  there  was  no  mischief 
taking  place,  and  that  the  work  was  in  a 
safe  condition.  There  is  no  doubt  that 
embankments  in  this  condition  require  to 
be  narrowly  watched,  although  the  pre- 
sumption may  be  that,  having  lasted  for 
several  years,  they  will  continue  in  safety. 

The  empirical  and  unscientific  mode  of 
proportioning  the  length  of  waste  weirs 
has  proved  before  now  a  source  of  danger 
and  destruction  to  embankments,  from 
the  space  afforded  not  being  sufficient  to 
discharge  the  excess  water  without  the 
surface  rising  to  such  a  height  as  to  top 
the  embankment.  To  avoid  risk,  the 
stream  must  be  gauged  with  great  care, 
and  the  discharge  calculated  for  the 
greatest  known  flood ;  and  if  with  a 
given  head  the  length  of  the  weir  be 
adjusted  to  discharge  this  amount,  or  a 
little  in  excess,  there  will  be  no  risk  to 
the  embankment. 

Regarding  finally  the  whole  subject, 
the  danger  that  may  result  from  careless 
or  unscientific  construction,  the  large 
outlay  entailed  in  the  establishment  of 


97 

storage  works,  and  the  benefit  that  may 
accrue  from  them  whatever  their  purpose 
may  be,  the  subject  cannot  be  under- 
taken on  merely  rational  grounds.  Its 
successful  application  will  rest  alone  on 
the  study  of  the  question  in  its  scientific 
details,  and  an  ample  practical  experi- 
ence. 

DISCUSSION. 

Mr.  H.  P.  Stephenson  said  he  entirely 
agreed  with  the  author  as  to  the  impro- 
priety of  carrying  a  pipe  through  the 
embankment  of  a  reservoir.  He  would 
extend  his  objection  to  the  passing  of  a 
culvert  through  the  embankment.  If  the 
culvert  were  laid  on  the  natural  ground, 
they  would  avoid  the  risks  pointed  out 
by  the  author,  either  of  the  settlement 
from  the  joints  of  the  pipe,  or  of  the 
water  creeping  along  between  the  ma- 
terial and  the  pipe.  He  believed  that 
the  true  principle  of  construction  for 
reservoirs  was  the  placing  of  a  good 
puddle  dam  in  the  center,  and  on  each 
side  of  this  dam  layers  of  earth  well 
punned  in.     One  reason  why  he  should 


98 


prefer  the  puddle  wall  in  the  center  was 
that  there  was  less  tendency  in  the  pud- 
dle to  slip  in  such  a  position  than  when 
laid  on  the  slope. 

Mr.  Albert  Latham  agreed  with  Mr. 
Stephenson  in  his  remarks  as  to  the 
pipes  and  culverts ;  but  he  thought  it 
was  an  open  question  whether  the  puddle 
wall  should  be  in  the  center  of  the  dam. 
He  had  a  strong  opinion  that  it  should 
be  on  the  face  of  the  dam. 

Mr.  Cargill  said  that  he  believed  that 
the  reason  the  puddle  wall  was  not  re- 
quired in  Indian  embankments,  referred 
to  by  the  author  of  the  paper,  was  that 
the  earth  seemed  to  have  been  thor- 
oughly consolidated  by  the  continual 
trample  of  people  upon  it.  That  thor- 
ough-consolidation was  the  great  point 
in  all  puddling,  and  it  was  on  that  ac- 
count that  specifications  were  generally 
so  stringent  as  to  the  thickness  of  the 
layers  of  the  puddle.  As  to  the  position 
of  the  puddle  wall,  he  could  not  see  the 
particular  value  of  having  it  in  the 
middle  of  the  dam,  and  he  thought  that 


99 


a  far  better  place  for  it  would  be  the 
face,  because  the  object  of  the  puddle 
wall  was  to  prevent  the  infiltration  or 
the  escape  of  the  water.*  This  could  be 
effected  by  puddling  the  whole  slope 
right  down  to  the  permanent  strata. 
The  puddle  wall  was  not  required  to 
promote  the  stability  of  the  dam.  The 
question  of  putting  pipes  or  culverts 
under  the  dam  required  more  considera- 
tion. It  was  alleged  that  the  putting  of 
a  naked  pipe  through  the  dam  of  the 
Bradfield  reservoir  was  one  of  the  causes 
of  its  bursting.  In  some  very  large, 
waterworks  now  being  constructed  in 
Dublin  there  were  two  distinct  sets  of 
main  pipes,  and  they  were  laid  in  two 
large  culverts  at  the  bottom  of  the  dam. 
The  culverts  were  large  enough  for  a 
man  to  walk  upright  in  them.  If  the 
foundation  were  well  looked  after,  there 

*.Asthe  center  wall,  of  puddle  or  masonry,  is  the 
citaael  of  the  dam,  it  would  seem  clear  that  it  sbould 
be  protected  by  being  placed  within  the  embankment. 
Placing  the  puddle  on  the  outside,  in  the  shape  of  a 
face  covering,  would  seem  to  invite  such  a  disaster 
as  that  mentioned  a«.  having  occurred  at  Sheffield. 


100 

would  be  no  fear  of  the  arch  or  dome  of 
the  culvert  giving  way  in  consequence 
of  any  inequality  of  pressure  above  it,  as, 
if  proi3erly  constructed,  an  arch  would 
stand  any  amount  of  pressure  short  of 
what  would  crush  the  material. 

Mr.  Baldwin  Latham  said  he  could 
not  agree  with  Mr.  Jacob  that  a  dam 
could  not  be  constructed  from  theo- 
retical deductions  ;  for  unless  regard  was 
paid  to  theoretical  considerations  there 
might  result  either  a  deficiency  of 
strength  or  a  waste  of  material  and 
labor.  In  the  dam  shown  in  the  draw- 
ings, and  designed  by  himself,  the  pipe 
did  not  run  through,  but  on  the  outside 
of  the  dam,  on  the  solid  ground.  It  was 
a  well  received  opinion  among  enghieers 
that  if  you  had  a  pipe  or  culvert  running 
through  an  embankment,  that  pipe  or 
culvert  would  be  unsafe.  He  believed 
that  well  made  and  properly  tested  pipes 
were  quite  as  safe  as  culverts  when  in 
the  solid  ground.  A  pipe  was  simply  a 
small  culvert  made  of  iron  instead  of 
brickwork.     In  cases  in  which  there  was 


101 

a  tendency  for  the  water  to  creep  along 
the  outside  of  the  pipe,  that  might  be 
stopped  by  having  projecting  flanges  on 
the  pipe.  The  same  creeping  of  water 
might  take  place  along  a  culvert  as  along 
a  pipe.  With  regard  to  the  slope  of  a 
dam,  the  inside  slope  should  be  greater 
than  the  outside  slope,  because  the 
greater  would  be  the  stability  of  the 
dam,  and  the  water  would  have  less  de- 
structive effect  on  the  dam ;  he  had 
effectually  prevented  leakage  by  the  use 
of  socket-pipes.  The  square  projection 
of  the  sockets  was  always  presented  to 
the  reservoir,  and  the  pipes  were  laid  in 
the  virgin  ground.  It  was  very  bad 
practice  to  lay  the  pipes  in  made  ground, 
and  especially  through  a  dam.  Pipes 
laid  under  a  dam  should  be  tested  under 
pressure  after  being  laid  and  before 
being  covered  up,  so  that  any  defective 
joint  might  be  discovered.  In  cases  in 
which  he  had  laid  pipes  through  dams, 
they  had  been  so  tested,  which  resulted 
in  good  and  effective  work ;  but  he  was 
bound  to  say  that,  if  the  pipes  had  not 


102 

been  tested  in  situ  the  result  would  not 
have  been  satisfactory. 

Mr.  Scbonheyder  said  that  Mr.  Jacob 
had  said  that  wherever  springs  occurred 
they  should  be  well  carried  away.  He 
(Mr.  Schouheyder)  wished  to  know  how 
a  spring  was  to  be  prevented  from  dif- 
fusing through  the  earth. 

Mr.  Hendry  said  that  he  had  seen  pipes 
which  were  laid  through  embankments, 
but  had  never  seen  one  that  was  per- 
fectly tight.  It  was  almost  impracticable 
to  make  it  so,  owing  to  the  continuity  of 
the  puddle  being  disturbed  at  the  point 
where  the  pipe  passes  through. 

The  chairman  asked  what  was  the 
largest  diameter  of  pipe  Mr.  Hendry  had 
seen  used. 

Mr.  Hendry  replied  that  the  largest 
was  18  in.  He  had  heard  of  several 
methods  being  tried,  but  he  did  not 
think  it  was  possible  to  prevent  leak- 
ing, more  or  less,  from  the  leservoir 
along  the  outside  of  the  pipe.  He 
should  like  to  be  informed  how  it  was 
possible  to  connect  the  puddle   with   the 


103 


pipe ;  if  the  pipes  be  laid  in  the  natural 
ground  below  the  foundation  of  the  em- 
bankment, then  there  is  no  fear  of  leak- 
age, provided  the  pipes  are  properly  laid. 
Mr.  Jacob,  in  replying  to  the  discus- 
sion, said,  that  in  the  opinions  that  had 
been  expressed  there  were  but  few  jooints 
of  disagreement  with  those  that  he  him- 
self held.  He  could  not  agree  with  Mr. 
Latham  in  his  belief  that  embankments 
could  be  calculated  on  mathematical 
principles.  In  order  to  deal  with  em- 
bankments theoretically,  they  must  be 
regarded  as  rigid  masses,  and  be  assumed 
to  rest  upon  a  horizontal  plane.  It 
could  be  shown  mathematically  that  a 
rigid  body  of  the  same  specific  gravity 
as  ordinary  earth  need  not  present  the 
same  section  as  is  usually  given  to 
embankments,  in  order  adequately  to 
resist  the  pressure  of  water.  A  right- 
angle  prism  with  the  hypothenuse  rest- 
ing upon  the  plane  would  be  quite  suffi- 
cient to  resist  the  pressure  of  water, 
even  supposing  the  surface  of  the  water 
to  coincide  with  the  upper   edge  of  the 


104 

prism.  The  reason  of  giving  long  slopes 
to  an  embankment  is  discoverable  from 
the  fact  that  banks,  when  exposed  to  the 
action  of  water,  are  found  to  waste  and 
slip  away  to  such  an  angle  as  will  with- 
stand the  action  of  the  water.  The  chief 
reason  of  the  failure  of  embankments  is 
the  infiltration  or  soaking  of  the  water 
from  the  inner  side,  which  renders  the 
material  semi-fluid  and  causes  it  to  sub- 
side into  a  smaller  space  than  it  origin- 
ally occupied.  The  superincumbent 
mass  then  sinks  and  allows  the  water 
to  overtop  the  embankment.  The  earth 
used  for  making  embankments  in  the 
Deccan  and  in  parts  of  the  Madras  Presi- 
dency in  India  is  of  a  most  suitable 
quality  for  the  purpose.  It  is  what  is 
called  "  black  soil,"  being  very  dark  in 
color,  and  of  a  highly  argillaceous  charac- 
ter. The  color  is,  no  doubt,  due  to  the 
presence  of  carbon.  The  clay  makes 
most  excellent  puddle  ;  but,  no  doubt,  the 
consolidation  produced  by  the  tread  of 
the  work-people  is  the  real  secret  of  the 
earth  resisting  the  pressure  of  water  so 


105 

successfully  as  it  does.  In  North 
America,  the  levees  for  protecting  the 
country  from  flooding  by  the  Mississippi 
are  sometimes  constructed  simply  of  sand; 
and  are  found,  for  the  most  part,  suffi- 
cient for  their  purpose.  A s  regards  carry- 
ing away  springs  from  the  seat  of  an  em- 
bankment, there  is  no  difficulty  in  ascer- 
taining where  they  exist  when  the  ground 
is  laid  bare,  as  they  are  generally  well- 
defined  streams.  Before  the  earthwork 
is  commenced  it  is  necessary  to  construct 
drains  of  masonry,  or  brickwork,  or  to 
lay  iron  piping  to  carry  away  the  water 
clear  of  the  work.* 

The  chairman  said  that  the  paper  of 
Mr.  Jacob  was  a  very  interesting  one, 
and  the  subject  was  one  which,  during 
the  last  year  or  two,  or,  he  might  say, 
within  the  last  week  or  two,  had  com- 
manded the  attention  of  the  whole  body  of 
engineers.  Last  session  a  special  Act  of 
Parliament  was  passed,  that  all  reservoirs 
and  embankments  should  be  constructed 

*  We  are  again  compelled  to  ask :    Where  to  ? 


106 

to  tbe  approval  of  the  Board  of  Trade. 
The  subject  of  irrigation  in  India,  which 
was  alluded  to  in  the  paper,  was  one  of 
vital  importance.  There  was  no  question 
that  the  only  means  we  had  of  irrigating 
that  country  in  an  efficient  manner  was 
,by  the  construction  of  reservoirs. 


107 


Additional   Remarks    by   the   American 
Editor. 


The  importance  of  storage  reservoirs 
for  the  purpose  of  equalizing  the  flow  of 
water  furnished  by  streams,  is  so  gener- 
ally recognized  that  they  must  be  con- 
sidered as  forming  essential  parts  of  all 
well-planned  systems  of  water  supply, 
excepting  those  maintained  by  bodies  of 
water  of  such  large  relative  dimensions 
that  their  minimum  flow  in  the  driest 
seasons  exceeds  the  maximum  con- 
sumption. 

The  admirable  paper  of  Mr.  Jacob, 
which  forms  the  first  part  of  the  present 
volume,  covers  the  ground  embraced  by 
its  title,  and,  together  with  the  subse- 
quent discussion,  forms  a  body  of  infor- 
mation of  the  highest  interest  and  value. 
The  art  of  hydraulic  engineering,  how- 
ever, is  a  progressive  one,  and  at  the 
present  day  there  is  much  which  may  be 


108 

added  to  the  original  paper,  particularly 
from  the  point  of  view  of  American  en- 
gineering, which  it  may  be  fairly  claimed 
is  likely  to  enhance  its  usefulness. 

In  planning  storage  reservoirs,  there 
are  two  questions  which  naturally  com- 
mand attention  from  the  very  start: 
How  much  stored  water  does  our  supply 
require  ?  And  how  much  can  our  available 
resources  be  counted  upon  to  furnish  ? 
In  answer  to  the  first  of  these  questions, 
I  cannot  probably  do  better  than  quote 
the  following  statement  made  by  Mr. 
Pole  (Proceedings  of  the  Institution  of 
Civil  Engineers,  1884-5)  as  regards  Eng- 
land: "The  general  judgment  of  ex- 
perienced practitioners  appears  to  be, 
that  for  large  rainfalls,  a  storage  of  150 
days'  supply,  or  even  less,  will  sufi&ce ; 
but  in  drier  districts  it  may  be  necessary 
to  go  as  high  as  200  days."  This  rule 
will,  I  think,  hold  good  for  this  country 
also,  and  we  may,  as  a  general  thing,  con- 
sider that  a  water  supply  which  compre- 
hends a  storage  equivalent  to  150  days' 
consumption,  is  in  a  good  position   for 


109 

the  maintenance  of  a  constant  supply  at 
all  seasons  in  districts  enjoying  an  aver- 
age rainfall.  It  will  be  understood  that 
this  amount  of  storage  applies  to  cases 
where  it  is  proposed  to  use  the  whole, 
or  greater  part,  of  the  total  yield  of  the 
stream. 

As  to  the  second  question,  the  answer 
depends  upon  the  minimum  annual  rain- 
fall over  the  basin  drained  by  the 
given  stream,  and  the  amount  of  this 
rainfall— deduction  made  for  losses  by 
evaporation,  percolation,  etc.,  as  well  as 
for  consumption,  which  is  available  for 
storage.  In  regard  to  this  matter,  Mr. 
Jacob  dwells  upon  the  necessity  of  ascer- 
taining the  amount  of  annual  rainfall, 
and  also  of  carefully  gauging  the  stream, 
in  order  to  establish  its  discharge  at  dif- 
ferent seasons  of  the  year.  Now,  it  is 
evident  that  these  operations,  to  be  of  any 
real  value,  require  a  long  period  of  time 
for  their  accomplishment,  and  can  be 
therefore,  in  the  great  majority  of  cases, 
of  only  pai'tial  utility.  Fortunately,  we 
can  frequently  dispense   with  any  con- 


110 

sideration  of  this  point,  for  our  stream 
may  be  of  such  size,  and  may  drain  so 
large  an  area,  that  we  need  not  be  at  all 
troubled  as  to  its  ability  to  fill  our  j^ro- 
posed  reservoir  to  the  desired  capacity. 
On  the  other  hand,  however,  cases  often 
present  themselves  when  it  becomes  a 
question,  if  we  build  a  reservoir  to  con- 
tain the  required  amount  of  water, 
whether  we  can  be  sure  of  filling  it  every 
year.  It  is  not  safe,  as  a  general  rule, 
and  in  average  locations  in  this  country, 
to  count  on  more  than  12  inches  of  avail- 
able annual  rainfall,  for  storage  purposes, 
and  even  this  limit,  if  great  interests  are 
at  stake,  should  be  approached  with 
caution.  Twelve  inches  of  rainfall  will 
furnish  27,878,400  cubic  feet,  or  a  little 
more  than  208.5  million  U.  S.  gallons  per 
square  mile  of  drainage  area,  and,  for  a 
round  number,  200  million  gallons  per 
square  mile  may  be  considered  as  the  maxi- 
mum that  it  would  be  safe  to  count  on, 
although  it  is  very  probable  that  there 
would  be  times  every  year  when  water 
ran  to  waste  over  the  spill- way,  indicat- 


Ill 

ing  that  the  capacity  of  the  reservoir  was 
not  as  great  as  the  flow  of  the  stream 
would  warrant.  It  might  therefore  be 
wise,  in  cases  where  it  was  important  to 
store  every  available  gallon,  and  worth 
while  to  spend  a  good  deal  of  money  to 
make  sure  of  doing  so,  to  increase  the 
relative  capacity  of  the  reservoir. 

In  the  Ci'oton  basin  the  average  yearly 
precipitation  is  almost  exactly  46  inches. 
The  very  careful  observations  made  by  the 
Croton  Aqueduct  Department,  and  ex- 
tending over  a  long  period  of  years,  indi- 
cate that  in  this  basin  each  square  mile 
of  drainage  area  will  furnish  an  average 
water  supply  of  at  least  one  million 
gallons  per  day.  In  districts  of  similar 
geological  character,  and  with  equal 
average  yearly  rainfalls,  the  same  rule 
may  be  considered  to  hold  good.  Under 
such  circumstances,  a  reservoir  capacity 
of  200  million  gallons  per  square  mile 
would  furnish  storage  for  200  days' 
supply. 

It  will  be  seen  that  the  three  points 
governing  the  question  of  storage  capa-- 


112 

city  are  — the  drainage  area,  the  available 
rainfall,  and  the  daily  supply  which  it  is 
desirable  to  maintain. 

These  preliminary  questions  being 
settled,  the  next  point  coming  up  for 
decision  will  be,  the  best  location  for  the 
dam  which  is  to  form  the  reservoir.  The 
approximate  location  can  generally  be 
readily  selected :  The  natural  features 
of  the  ground  usually  clearly  indicate  the 
point  near  which  the  dam  should  be  built. 
In  order  to  determine  the  exact  position, 
the  whole  probable  area  should  be  cross- 
sectioned  in  20  feet  squares  (with  addi- 
tional points  where  needed).  This  work 
will  not  only  show  the  precise  line  where 
the  longitudinal  section  of  the  dam  has 
the  smallest  area,  which,  other  things 
being  equal,  will  be  the  best  line,  but 
will  also  preserve  a  record  of  the  original 
surface  of  the  ground,  from  which  the 
amount  of  work  done  at  any  time  can  be 
readily  estimated. 

The  height  and  location  of  the  dam 
being  thus  settled,  the  next  important 
question  is,  What  sort  of  a  dam  shall  we 


113 


build  ?  It  will  be  observed  that  in  Mr. 
Jacob's  paper,  he  confines  himself  to  the 
consideration  of  earthen  dams  only. 
These  dams  can  usually  be  built  cheaper 
than  masonry  dams,  and  have  this  ad- 
vantage, that  they  admit  of  being  built 
in  places  where  masonry  dams  would  be 
unsafe.  It  will  very  seldom  be  found 
advisable  to  build  a  high  masonry  dam 
on  anything  but  a  foundation  of  solid 
rock.  When  such  natural  foundation  is 
not  found,  and  it  is  still  determined  to 
build  a  masonry  dam,  the  only  resource 
in  order  to  secure  safety,  is  to  carry  the 
foundations  down  to  a  depth  which  will 
very  greatly  increase  the  cost  of  the 
work. 

The  dams  spoken  of  by  Mr.  Jacob  are 
of  a  type  very  prevalent  in  England  and 
India :  namely,  earthen  dams  with  a 
puddle  core.  Such  dams  are  rare  in  the 
United  States,  where  the  puddle  core  is 
commonly  replaced  by  a  masonry  center 
wall.  The  object  in  either  case  is, 
primarily,  to  establish  an  impervious  cut- 
off   against     any    possible     percolation 


114 

through  or  under  the  earthern  embank- 
ment. I  think  there  can  be  no  doubt  as 
to  the  superior  efficiency  of  the  masonry 
wall,  and  I  doubt  if  there  is  as  great  an 
economy  in  the  puddle,  as  would  at  first 
appear  probable.  In  the  first  place, 
proper  material  for  good  puddle  is  not 
always  obtainable,  and  even  when  it  is 
found  in  abundance  near  by,  its  proper 
preparation  and  placing  are  matters  re- 
quiring a  good  deal  of  careful  and  ex- 
pensive manipulation.  Good  puddle 
should  resemble,  when  made  and  placed, 
in  character  and  composition,  an  unburnt 
brick.  When  we  read,  as  we  do  in  Mr. 
Jacob's  paper,  and  elsewhere,  of  the  great 
precautions  necessary  in  putting  in  a 
puddle  wall,  and  the  disastrous  conse- 
quences attendant  upon  some  appar- 
ently trifling  neglect  in  doing  so,  I  think 
most  engineers  intrusted  with  the  design- 
ing of  so  important  a  work  as  a  large 
storage  reservoir,  would  hesitate  before 
risking  these  consequences  in  order  to 
effect  an  economy  which  perhaps  might 
eventually  prove  to  be  not  so  great  as 
was  anticipated. 


115 

There  is  another  particular  in  which 
the  masonry  center  wall  presents  a  great 
superiority  to  the  puddle  core.  One  of 
the  weakest  points  of  an  earthen  dam  lies 
in  the  neighborhood  of  the  conduit  used 
for  drawing  off  the  water.  A  perusal  of 
Mr.  Jacob's  paper  and  the  discussion 
shows  what  importance  is  attached  to 
having  this  conduit  form  a  perfectly  water- 
tight connection  between  the  inside  and 
outside  of  the  reservoir.  It  is  clear  that 
the  existence  of  solid  and  water-tight 
masonry  center  wall,  running  through 
the  entire  length  of  the  dam,  and  firmly 
and  deeply  imbedded  in  the  banks  of  the 
valley  on  each  side,  affords  an  excellent 
opportunity  for  making  all  the  necessary 
connections  in  a  satisfactory  manner. 
The  masonry  culvert  through  which  the 
water  passes,  and  the  gallery  containing 
the  pipes,  may  be  bonded  in  with  the 
center  wall  and  made  to  form  a  part  of  it 
in  such  a  way  that  the  possibility  of  water 
following  along  the  outside  of  these 
structures  is  wholly  precluded,  and  one 
chief  danger  of  destruction  of  the  dam 
entirely  averted. 


116 

Having  now  decided  upon  the  character 
of  our  proposed  dam  ;  viz : — an  earthen 
dam  with  masonry  center  wall,  or  cut  off, 
it  is  next  in  order  to  consider  its  design, 
dimensions  and  accessories. 

As  regards  the  latter,  the  spill-way 
merits  the  first  mention.  The  dimen- 
sions of  this  portion  of  the  dam  must  be 
ample,  and  sufficient  to  safely  pass  all  the 
water  which  may  come  to  it  in  times  of 
heaviest  freshet,  without  the  possibility 
of  its  overtopping  the  dam.  As  to  the 
proper  length  of  the  spill-way,  it  is  per- 
haps impossible  to  lay  down  any  fixed 
rule,  or  to  attempt  to  make  it  a  given 
function  of  the  water  shed.*  If,  in  design- 
ing a  dam,  we  find  any  existing  dams  in 
the  neighborhood,  upon  the  same  stream, 
or,  failing  this  best  indication,  any  rail- 
road bridges  through  which  the  whole  of 
the  stream  has  to  pass,  we  may  have  a 
good  opportunity  of  ascertaining  the 
amount  of  water  passing  off  in  freshets, 
and  proportion  our  spill- way  accordingly. 
If  none  of   these  indications  are   to  be 

*  See  page  S7. 


117 

found,  the  next  best  course  to  pursue 
(and  this  should  be  done  in  any  event, 
as  a  check),  is  to  ascertain  the  dimensions 
of  the  spill- way  of  some  existing  dam, 
built  elsewhere,  but  upon  a  stream  of 
about  the  same  drainage  area  as  the  one 
in  question,  and,  if  possible,  of  the  same 
character. 

"When  a  natural  spill-way  cannot  be 
found,  such  as  Mr.  Jacob  speaks  of,  and 
which,  of  course,  is  always  preferable,  its 
construction  of  stone  masonry  is  always 
a  very  expensive  piece  of  work,  and  the 
tendency  is  therefore  to  reduce  the  length, 
and  provide  for  a  deep  wave  passing  over 
it.  This  kind  of  economy  should  not  be 
pushed  too  far ;  it  is  better  and  safer  to 
provide  a  long  spill-waj^  over  which  a 
comparatively  thin  sheet  of  water  shall 
pass. 

The  dimensions  of  this  important  part 
of  our  dam  having  been  settled,  the  next 
point  to  consider  is  what  relative 
position  it  should  occupy.  If  the  sides 
of  the  valley  are  of  rock,  even  if  not  of 
the  very  soundest  and  most  solid  char- 


118 

acter,  there  will  be  an  economy  in  placing 
the  spill-way  at  one  end  of  the  dam,  as 
its  height  will  thereby  be  diminished.  If, 
however,  the  sides  are  of  earth,  the  safest 
place  for  it  is  directly  in  line  with  the 
stream,  as  it  would  be  dangerous  to  dis- 
charge a  large  volume  of  water  upon  the 
unprotected  hillside,  and  any  attempt  to 
protect  it  against  wash,  by  means  of  walls 
and  paving,  will  generally  involve  an  ex- 
pense equal,  or  nearly  so,  to  the  higher 
structure.  This  is  a  point,  however, 
which  is  by  no  means  to  be  taken  for 
granted,  and  in  cases  where  great 
economy  of  construction  is  necessary,  the 
ground  should  be  carefully  examined,and 
comparative  estimates  made. 

As  regards  the  form  of  the  spill-way, 
the  curved  form  adopted  for  the  old 
Croton  dam,  and  the  dam  on  the  Bronx, 
at  Kensico,  N.  Y.,  is  no  doubt  the  best, 
but  its  great  cost  in  the  way  of  cut  stone 
voussoirs  will  generally  preclude  its  use 
except  for  municipal  work,  and  where 
economy  is  studied,  the  form  adopted 
will  generally  be  that  of  steps,  or  offsets. 


119 

A  good  form  of  spill- way  is  shown  in  Fig. 
72,  page  382,  of  Tanning's  "  Water  Sup- 
ply Engineering."  A  very  good  and 
massive  form,  which  I  have  had  occasion 
to  adopt  with  some  modifications  for  a 
spill- way  about  30  feet  high,  is  described 
as  follows  :  Back,  vertical ;  top  width, 
7  feet ;  batter  on  face,  an  inch  and  a  half 
to  the  foot,  for  8  feet,  making  a  thickness 
of  8  feet  at  the  bottom.  Thence,  steps 
ranging  in  height  from  15  to  22  inches, 
and  following  a  general  slope  of  45  de- 
grees. In  this  way,  it  will  be  perceived 
that  at  any  given  elevation,  the  thickness 
of  the  wall  is  always  equal  to  its  height 
above  such  elevation. 

The  waste  pits  and  sluices  spoken  of 
by  Mr.  Jacob  as  substitutes  for  a  waste 
weir,  are  not,  I  think,  to  be  recom- 
mended. Nothing  can  be  better  than  a 
straight  opening  and  clear  escape  for  the 
surplus  water. 

As  regards  the  means  adopted  for 
drawing  off  the  water,  they  should  be  ns 
simple  as  possible,  with  good  facilities  f  )r 
inspection  and  repair.     Those  menti^^f  I 


120 

by  Mr.  Jacob  seem  to  be  unnecessarily 
complicated.  My  own  opinion  is  strongly 
in  favor  of  having  a  cast-iron  pipe  or 
pipes  built  into  the  center  wall,  and  ex- 
tending outside  of  the  dam,  in  an  arched 
gallery,  founded  in  the  natural  formation 
sufficiently  large  to  admit  of  free  circula- 
tion all  about  the  pipes.  Within  this  gal- 
lery, or  gate  chamber,  are  the  gates,  two 
upon  each  pipe,  the  inner  one,  or  one 
nearest  the  water,  to  be  kept  habitually 
open,  and  the  delivery  of  water  regulated 
by  means  of  the  outer  one.  If  any  accident 
occurs  to  this  gate,  the  inner  one  is 
closed,  and  it  can  then  be  got  at  for  re- 
pairs. On  the  inside,  a  tower  is  built, 
one  side  of  which  is  formed  by  the  center 
wall  through  which  the  pipes  run.  This 
tower,  or  well,  is  rectangular  in  shape, 
and  its  sides  are  from  one  foot  to 
eighteen  inches  outside  of  the  pipes ; 
that  is,  its  inside  width  is  from  2  to  3 
feet  greater  than  the  outside  diameter  of 
tli(^  reducer  of  the  largest  pipe,  so  as  to 
afford  a  chance  to  lead  and  calk  the  re- 
ducer when  set  in    the   cut    ring   stones 


121 

which  surround  it.  The  length  of  the 
tower  may  be  from  10  to  15  feet.  It  con- 
tains two  sets  of  cut  stone  grooves,  in  which 
stop-plank  can  be  placed.  By  this  means, 
should  it  ever  become  desirable  to  get  at 
the  mouth  of  the  pipes  or  reducers  with- 
out first  emptying  the  reservoir,  the  stop- 
plank  can  be  placed,  and,  if  necessary,  the 
space  between  them  puddled,  and  a  water- 
tight coffer  dam  is  thus  made,  inside  of 
which  work  can  be  carried  on  while  the 
reservoir  is  full  of  water.  From  this 
tower,  a  masonry  culvert,  also  provided 
with  stop-plank  tower,  if  thought  neces- 
sary, or  an  open  passage,  with  wing 
walls,  extends  through  the  embankment. 
The  gate  chamber,  tower,  wing  walls  and 
spill- way  should  be,  if  possible,  grouped 
together,  with  a  view  to  economy  of  ma- 
terials and  increased  strength,  and  an 
effort  should  be  made  to  so  locate  the 
work  that  these  important  features  should 
be  set  upon  the  most  favorable  natural 
foundation. 

As  regards  the  center  wall  of  masonry, 
it  is,  of   course,  impossible  to  lay  down 


122 

hard  and  fast  rules  for  its  proportions, 
any  more  than  for  the  other  parts  of  the 
dam.  But  certain  general  principles 
may  be  established,  to  do  which  it  is  first 
necessary  to  get  a  clear  idea  of  the 
precise  function  of  the  center  wall,  and 
of  the  part  it  is  to  play  in  the  dam.  As 
we  have  seen,  it  is  intended,  primarily,  to 
afford  a  water-tight  cut-off,  to  arrest 
any  percolations  which  may  reach  it,  by 
trickling  through  the  bank.  Indeed,  we 
may  consider  the  center  wall  as  constitut- 
ing the  dam  proper,  for  it  is  to  the  center 
wall  that  we  finally  look  for  the  retention 
of  the  water  within  the  reservoir.  Re- 
garded in  this  way,  the  earthen  embank- 
ments on  each  side  are  only  provisions 
for  keeping  the  center  wall  from  being 
thrown  down.  In  point  of  fact,  however, 
we  do  expect  more  than  this  from  the 
embankments.  We  expect  the  inner  em- 
bankment to  be  very  nearly  impervious, 
and  of  itself  to  be  almost,  if  not  quite, 
suflScient  for  the  retention  of  the  water. 
In  any  event,  we  expect  it  to  be  a  power- 
ful auxiliary  to  the  center  wall,  by  keep- 


123 

ing  the  deepest  water  well  back  from  it, 
and  increasing  the  distance  that  a  given 
drop  of  water  would  be  obliged  to  travel 
in  order  to  pass  underneath  the  wall.  In 
this  way,  the  embankment  is  equivalent 
to  a  deepening  of  the  foundation  of 
the  wall.  Moreover,  water  reaching 
the  center  wall  by  traversing  the  em- 
bankment, comes  to  it  in  the  form 
of  a  percolation,  modified  by  capillary 
action.  The  object  of  the  exterior  bank 
is  mainly  to  keep  the  wall  from  being 
thrown  out,  but  it  serves  an  excellent 
purpose  also  in  smotheiing  down  any 
slight  percolations  issuing  from  under- 
neath the  wall,  and  still  further  increases 
the  distance  that  a  given  drop  of  water 
must  travel  in  order  to  pass  freely  from 
the  inside  to  the  outside  of  the  reservoir. 
As  to  the  proper  height  to  give  to  the 
center  wall,  although  it  is  highly  probable 
that  in  many  cases  it  is  not  necessary  to 
carry  it  up  to  the  full  height  of  the  sur- 
face of  the  water,  when  the  reservoir  is 
full,  we  cannot  say  that  an  earthen  dam  is 
absolutelv  safe  unless  the  wall  is  carried 


124 

at  least  as  high  as  the  Up  of  the  spill- 
way. It  is  still  better,  in  very  high  dams, 
to  raise  it  as  high  as  the  extreme  eleva- 
tion of  flood  freshets.  As  to  its  thick- 
ness, economy  will  prompt  a  reduction  to 
the  narrowest  limits,  but  it  must  not  be 
forgotten  that,  although  it  depends  on  the 
banks  for  support,  rather  than  to  its  own 
moment  of  inertia,  yet,  owing  to  the  fact 
that  some  movement  may  take  place  in 
the  banks  themselves,  it  is  liable  to  be 
subjected  to  unbalanced  pressures,  and 
should  therefore  be,  to  some  extent,  self- 
supporting.  If  we  start  with  a  top  thick- 
ness of  5  feet,  and  increase  the  thickness, 
by  off-sets,  to  the  extent  of  2  feet  addi- 
tional, every  10  feet  as  we  go  down,  we 
shall  have  dimensions  which,  in  a  great 
majority  of  cases,  will  be  abundant  to 
afford  the  required  strength. 

A  still  more  important  question  in  re- 
gard to  the  center  wall,  is  the  depth  to 
which  its  foundations  should  be  carried. 
For  this  there  is  absolutely  no  rule, 
and  the  question  must  be  decided  accord- 
ing to  the  engineer's  best  judgment  for 


125 

each  particular  case.  Of  course,  if  a  rock 
foundatiou  is  encountered,  the  problem  is 
greatly  simplified,  for  we  have  only  to 
remove  the  loose  and  disintegrated  sur- 
face, until  we  come  to  clean,  live  rock, 
and  place  our  bottom  course  upon  that. 
The  uncertainty  occurs  when  our  test 
pits  reveal  the  existence  of  coarse  and 
permeable  strata,  when  our  only  resource 
is  to  carry  our  wall  well  down.  In  some 
cases  such  walls  have  been  put  down  to 
such  a  depth  that  in  places  the  distance 
below  ground  is  greater  than  the  height 
above,  and  frequently  sheet  pihng  has 
been  resorted  to.  This,  however,  it  is 
best  to  avoid,  if  possible. 

In  general,  it  may  be  said  that,  the 
finer  the  material,  the  better  it  is  adapted 
to  serve  as  a  footing  for  the  center  wall. 
Clay,  fine  gravel  and  sand,  and  quick- 
sand when  found  at  a  sufficient  depth  to 
prevent  its  being  forced  up  at  the  sides, 
are  all  excellent  materials  to  build  such  a 
wall  upon.  Where  a  compact,  water- 
tight material  is  found,  it  is  needless  to 
carry  the  footing  courses   to  any  great 


126 

depth  below  the  surface  of  the  ground, 
but  when  loose  cobbles,  coarse  gravel  and 
shingle,  or  any  other  ground  affording 
an  easy  passage  for  water,  is  encountered, 
the  only  safety  is  in  depth  of  foundation. 
It  now  remains  to  say  a  few  words 
respecting  the  earthen  embankments. 
On  page  85  Mr.  Jacob  gives  a  little  table 
of  the  proper  heights  for  the  top  of  em- 
bankments above  the  crest  of  the  spill- 
way, or  waste  weir.  It  is  very  important 
that  this  height  should  be  amply  suffi- 
cient to  prevent  the  embankment  being 
overtopped  during  a  freshet.  Of  course, 
the  length  of  spill-way  is  a  factor  in  this 
matter,  but  the  top  of  the  embankment 
should  be  kept  well  up,  and  I  should  be 
disposed  to  add  a  foot,  or  even  two,  to 
the  figures  given  by  Mr.  Jacob,  making 
them  respectively  5,  7  and  8  feet,  and 
I  would  change  the  maximum  from  10  to 
]  2  or  14  feet.  The  top  width  may  vary, 
according  to  the  height  of  the  dam,  from 
5  to  20  or  more  feet.  Perhaps  a  fair  ap- 
proximate rule  would  be,  to  give  the  top 
of  the  embankment   such   a   width   that, 


127 

taken  in  connection  with  the  height  above 
the  crest,  or  Hp  of  spill-way,  and  the 
slopes,  would  give  at  the  level  of  the  lip, 
a  thickness  at  least  equal  to  the  greatest 
depth  of  water  in  the  reservoir. 

For  the  slopes,  much  depends  upon 
the  material  used  for  embankments.  If 
a  good  quality  of  earth  is  to  be  found, 
such  as  a  sandy  loam,  containing  a  good 
proportion  of  clay  and  few  stones,  great 
thickness  of  embankment  is  not  necessary. 
If  the  quality  of  the  stuff  is  inferior,  it 
must  be  made  up  for  in  quantity.  Gener- 
ally slopes  of  from  2^  to  3^  to  1  for  the 
inside,  and  2J  to  1  for  the  outside  are 
found  sufficient.  An  excellent  profile  is 
obtained,  with  good  material,  by  starting 
from  the  top  with  a  slope  of  2^  to  1 
on  both  sides,  and  at  about  half-way 
down  introducing  a  level  berme  on  the 
inside,  from  which  a  slope  of  3  or  3^  to 
1  is  continued  down  to  the  foot  of  the 
embankment,  the  outside  slope  being 
kept  at  the  same  slope  of  2^  through- 
out. 

The  manner  in  which  the  embankment 


128 

is  formed,  is  a  matter  of  great  import- 
ance. It  is  necessary  to  put  it  in  in  such 
a  manner  as  to  minimize  the  subsequent 
settlement.  It  must  not,  therefore,  be 
carried  along  like  a  railroad  embank- 
ment, but  brought  up  in  horizontal 
layers  from  the  bottom.  Generally 
speaking,  if  the  material  be  brought  on 
in  carts  and  wagons,  and  kept  constantly 
moist  by  sprinkling,  the  travel  of  the 
vehicles  and  animals  will  be  suflScient  to 
secure  the  necessary  degree  of  compact- 
ness. The  face  of  the  interior  slope 
should  be  well  pitched,  or  rip-rapped,  as 
recommended  by  Mr.  Jacob,  and  the 
exterior  slope  sodded,  or  sown  to  grass. 
To  return  to  the  center  wall,  in  order 
to  say  a  few  words  as  to  the  manner  in 
which  it,  and  in  general  all  the  masonry 
work  of  the  dam,  should  be  executed. 
The  first  essential  of  all  hydraulic  ma- 
sonry is,  that  it  shall  be  as  nearly  as 
possible  water-tight.  In  order  to  effect 
this,  there  must  be  no  vacancies  what- 
ever between  any  two  contiguous  stones, 
but  the  wall   must  be  so  perfectly  laid 


129 

that  whatever  is  not  stone,  is  mortar,  and 
compact  mortar.  In  order  to  secure 
perfect  work,  the  stones  used  must  be 
clean  and  bright,  with  sharp  quarry- 
faces.  All  soiled  and  dirty  stones  should 
be,  if  possible,  discarded,  and  if  not,  they 
should  be  thoroughly  cleaned,  by  brooms 
or  brushes,  or  by  washing,  before  using, 
for  the  mortar  will  not  properly  adhere 
if  there  be  a  loose  coating  of  any  foreign 
substance  upon  the  surface  of  the  stone. 
If  necessary,  they  should  be  wet  before 
laying,  but  frequently,  particularly  when 
Portland  cement  is  used,  the  mortar 
throws  off  so  much  water  that  the  chief 
difficulty  is  to  keep  the  work  sufficiently 
dry.  When  a  piece  of  wall  is  to  be 
worked  upon,  it  should  be  carefully 
cleared  of  all  loose  and  dry  mortar;  in- 
deed, it  is  often  necessary  to  keep  men 
employed  constantly  in  sweeping  off  the 
wall  in  order  not  to  delay  the  masons  in 
cleaning  it  up.  This  point  should  always 
be  insisted  upon,  and  in  summer  there 
should  be  a  number  of  large  watering- 
pots  on  the  work,  to  sprinkle  the  surface 


130 

after  it  has  been  swept  off.  Great  care 
should  also  be  taken  to  have  the  mortar 
properly  mixed — a  point  too  often  neg- 
lected. The  sand  and  cement  should  be 
thoroughly  mixed  together  when  dry> 
by  repeated  turnings,  until  the  mass  has 
become  perfectly  homogeneous,  and  of 
an  uniform  color. 

It  is  generally  a  provision  of  the  speci- 
fications, that  stones  must  not  exceed  a 
certain  limit  in  size — perhaps  8  or  10 
cubic  feet — it  being  supposed  that  small 
stones  will  be  more  easily  bedded  than 
large  ones.  I  think  that  this  is  at  least 
doubtful,  and  that  stones  measuring  a 
cubic  yard,  or  even  a  yard  and  a  half,  can 
be  properly  bedded  with  very  little  addi- 
tional trouble.  The  gain  in  rapidity  of 
execution,  when  larger  stones  are  used,  is 
very  great,  as  a  derrick  can  swing  a  large 
stone  as  quickly  as  a  small  one,  and  there- 
fore many  more  yards  can  be  laid  in  a 
day.  Large  stones  also  bed  themselves 
more  perfectly,  from  the  greater  force 
with  which  they  compress  the  mortar 
under    them,    and  drive  out  the   super- 


131 

flaous  water.  Of  course,  stones  should 
not  be  allowed  of  such  dimensions  as  to 
run  through  the  wall  from  front  to  rear, 
and  it  is  always  well  to  reserve  the  right 
to  discard  stones  above,  say,  10  cubic 
feet,  by  a  clause  to  that  effect,  in  the 
specij&cations. 

After  a  stone  has  been  set,  it  should 
be  tested  with  bars,  so  as  to  see  if  it  is 
entirely  clear  of  everything,  and  swims 
on  its  bed,  without  rocking.  No  two 
stones,  however  small,  should  be  allowed 
to  touch  each  other  without  the  inter- 
vention of  compact  mortar.  When  there 
is  any  doubt  as  to  whether  a  stone  is 
properly  bedded,  it  should  be  raised,  so 
as  to  see  if  the  mortar  under  it  has  taken 
a  good  impression.  It  is  frequently 
necessary  to  raise  a  stone  a  second  and 
even  a  third  time,  before  it  can  be  pro- 
nounced to  be  satisfactorily  set,  particu- 
larly with  inexperienced  masons.  The 
masons  should,  however,  soon  become 
accustomed  to  properly  spreading  the 
beds,  so  that  a  majority  of  the  stones 
can  be  set  by  first  intention.     All  work 


132 

should  be  done  with  cranes  or  derricks, 
so  that  stones  can  be  dropped  in  their 
places,  and  readily  lifted  again,  when 
required.  All  spalls  and  small  stones 
should  be  settled  in  their  beds  by  a  blow 
with  the  hammer.  This  scrupulous  care 
in  laying  the  masonry  does  not  consume 
as  much  time  as  might  be  anticipated. 
There  should  be  no  trouble,  with  good 
and  shapely  stones,  in  laying  from  30  to 
35  cubic  yards  per  day,  per  derrick 
(steam),  with  say  six  masons  and  a  suffi- 
cient number  of  helpers,  and  yet  take  all 
the  precautions — and  many  more — above 
indicated.  Of  course,  to  effect  this,  the 
work  must  be  well  systematized,  and  the 
derricks  constantly  fed  with  material,  so 
that  the  masons  are  never  kept  waiting. 
In  putting  down  foundations,  care 
must  be  taken  to  provide  abundant 
pumping  power.  All  pumping  sumpts 
should  be  put  down  well  below  the 
bottom  of  the  trenches  they  are  designed 
to  drain,  otherwise  these  latter  will  never 
be  kept  properly  dry.  The  sumpts 
should  also,  if  possible,  be  placed  entirely 


133 


outside  the  lines  of  the  masonry,  so  as  to 
be  out  of  the  way,  and  not  draw  the  ma- 
terial from  under  work  already  laid. 

The  management  of  the  water  is  always 
one  of  the  greatest  difficulties  connected 
with  this  class  of  work.  The  foundations 
extend  entirely  across  the  valley  of  the 
stream,  and  provision  must  be  made  for 
the  passage  of  the  water  while  the  work 
is  in  progress.  Very  frequently,  partic- 
ularly in  the  case  of  small  streams,  the 
difficulty  can  be  met  by  first  building 
the  gate  chamber  and  setting  the  pipes 
and  gates,  and  then  turning  the  stream 
through  them,  thus  getting  rid  of  it  at 
once.  In  larger  streams  this  is  not 
always  practicable,  as  we  may  not  be 
sure  that  the  pipes  will  carry  all  the 
water  in  case  of  a  freshet.  In  such 
cases,  we  may  be  obliged  to  omit  setting 
the  pipes  till  after  the  completion  of  the 
work,  allowing  the  water  to  pass  through 
the  gate  chamber  and  gallery  in  which 
they  are  subsequently  to  be  set.  When 
this  course  is  pursued,  the  opening  in 
the  center  wall  must  be  so  planned  and 


184 

built  that  it  can  be  quickly  and  perfectly 
closed,  when  the  time  comes,  by  cut-stone 
pieces  carefully  dressed  to  fit.  When 
the  dam  is  otherwise  completed,  the  stop- 
plank  can  be  placed,  and  the  water  held 
back  while  the  pipes  and  gates  are  set. 
In  cases  where  the  stream  is  so  large 
that  even  these  means  are  not  sufficient, 
the  problem  becomes  exceedingly  diffi- 
cult, and  demands  very  careful  special 
study. 

Mr.  Jacob  dwells  much  upon  the 
danger  caused  by  the  existence  of  springs 
underneath  the  embankment,  and  recom- 
mends that  they  be  "taken  up  and  carried 
away  in  proper  drains  sufficiently  and 
securely  puddled."  Now,  unfortunately, 
this  is  easier  said  than  done.  The  site 
of  an  embankment  is  frequently  covered 
by  a  multitude  of  small  springs,  the 
sources  of  which  it  is  impossible  to 
locate,  and  to  which  they  cannot  be 
traced.  Besides,  where  should  we  carry 
them  to?  Probably  the  best  way  of 
dealing  with  such  springs,  is  to  get  the 
site    of     the     inner     embankment    well 


135 

stripped  down  to  the  live  earth,  and  then 
commence  placing  the  embankment  next 
to  the  center  wall  (not,  of  course,  carry- 
ing it  up  so  high  as  to  create  a  dump, 
such  as  we  have  already  seen  should  be 
avoided,)  and  then  advance  it  out  from 
the  wall,  toward  the  toe  of  the  slope,  in 
the  hopes  of  smothering  down  the 
springs,  and  driving  them,  to  some  ex- 
tent at  least,  back  from  under  the  em- 
bankment. Indeed,  it  is  difficult  to  see 
what  else  can  be  done,  and  it  may  be 
fairly  doubted  if  the  existence  of  such 
springs  is  fraught  with  such  serious 
menace  to  the  work  as  Mr.  Jacob  men- 
tions. Very  large  springs  might  occa- 
sion trouble,  if  the  weight  and  compact- 
ness of  the  bank  do  not  force  them  back, 
and,  if  possible,  such  should  be  traced  to 
their  source,  and  diverted  within  the 
reservoir.  Unfortunately,  these  large 
veins  of  water  are  frequently  fed  by  a 
number  of  separate  springs,  which  finally 
run  together,  and  if  the  flow  be  stopped 
in  one  point,  it  breaks  out  in  another. 
Large  springs,  such  as  have  been  just 


136 

ni(  ntioned,  often  give  great  trouble  in 
closing  the  gap  in  the  foundation  of  the 
center  wall,  which  has  been  left  for  them 
to  flow  through    temporarily,    the    diffi- 
culty being  of  the  same  class,  though  less 
in  degree,  as  that  occasioned  by  the  main 
stream  itself.     They  may  sometimes  be 
passed  over   from  side  to  side,  until   a 
point  is  reached,  above  which  they  do  not 
rise  in  any  considerable  volume.     Some- 
times they  are  best  handled  by  reducing 
the  width  of  the  gap  progressively,  until 
it  becomes  a  very  narrow  passage,   and 
then   closing  it  boldly,  working  in   the 
water  with  mortar  made  of  neat  cement, 
without  any  admixture  of  sand.     There 
can  be  no  rule  laid  dowQ  for  the  treat- 
ment   of    such    cases,   which    must    be 
handled  as  best  we  may,  the  only  thing 
necessary  and  sufficient  being  to  get  the 
wall  so  built  that  the  water   does  not, 
finally,    wash   out   the   mortar   and   run 
through  it.     If  we  fail  in  accomplishing 
this  in  one  way,  there  is  nothing  for  it 
but  to  take  out  the  defective  part  and 
try  some  other  means,  never  leaving  the 


137 

job  till  the  essential  object  aimed  at  has 
been  accomplished. 

In  conclusion,  it  may  be  said  that  the 
building  of  dams  and  reservoirs  belongs 
to  one  of  the  most  important  and  re- 
sponsible classes  of  work  undertaken  by 
the  engineer,  and  no  pains  should  be 
spared  in  their  construction  to  ensure 
satisfactory  results.  It  should  be  borne 
in  mind  that  the  first  essential  in  a  dam 
is  stability;  the  second  impermeability. 
At  the  present  day,  with  all  past  experi- 
ence as  a  guide,  there  should  be  no 
uncertainty  as  to  the  realization  of  the 
first  of  these  essentials ;  the  perfect  ful- 
fillment of  the  second  cannot  always  be 
so  surely  counted  upon,  because  it  de- 
pends not  only  upon  the  quality  of  our 
work — which  is  a  controllable  factor — 
but  also  upon  the  character  of  the  ground 
itself  on  which  the  work  is  placed,  which 
may  present  disadvantageous  features 
that  we  cannot  wholly  overcome.  But 
by  care  and  the  exercise  of  sound  judg- 
ment these  adverse  conditions  can  gener- 
ally be  modified  to  the  extent  of  securing 
satisfactory,  if  not  perfect  results. 


UCSB   LIBRARY 


FACIUTV 


'b    000  007  947     5 


